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This study performs the comprehensive analysis with regards to the amount of ultrafine WC grains needed for the appropriate reinforcement of the metallic binder in WC-8Co cemented carbides. The ratio of coarse versus ultrafine WC grains in the bimodal system is examined. The goal is to investigate the balance of grain distribution to achieve overall improvement of the material's mechanical and wear properties.

This work reports on hard part turning of carburizing steels using a PCBN cutting tool in fine machining. Emphasis is on the link between composition of the inclusions in work material and wear mechanisms of the cutting tool. A Ca-treated machinability improved 20NiCrMo steel was included together with three other carburizing steels with different inclusion characteristics. Machining tests were conducted to examine cutting tool life and its balance between excessive flank wear and crater wear. The wear mechanisms were examined using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a secondary electron (SE) detector. The longest tool life was obtained when cutting the Ca-treated steel. The improved machinability is linked to the deposition of complex (Mn,Ca)S and (Ca,Al)(O,S) protective slag layers that form on the rake face of the cutting tool during machining. Cutting in this steel also resulted in a typical ridge formation in the tool edge crater. Transfer of workpiece material to the rake face crater is characteristic in hard part turning of steel with high cleanliness. This is suggested to be related to the lack of the sulfides that lubricate conventional machinability treated steels, and that the crater wear of low-sulfur steel is more pronounced than for steels with higher sulfur content., Member research program in materials and manufacturing at Swerea KIMAB, Ovako and Sandvik Coromant

Curling is an Olympic winter sport in which two teams slide stones across a sheet of ice towards a target area, some 28 m away from the release line. The sport has its name from the fact that the trajectory of a rotating stone becomes slightly curled, a fact used to reach open spots or take out opponent stones behind hindering “guarding” stones, etc. By slowly turning the stone clockwise when it is released, it will curl to the right, and vice versa. The resulting sideward deviation is typically slightly more than a metre. This intriguing tribological phenomenon has so far lacked a satisfactory explanation, although many attempts have been presented. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In a recent paper, we could show that no such redistribution of the friction, no matter how extreme, can explain the magnitude of the observed motion of a real curling stone. The present work presents an alternative asymmetrical mechanism that actually is strong enough to account for the observed motion. Further, in contrast to previous models, it satisfies other observed phenomena, including the independence of rotational speed of the stone and the strong dependence of the roughness of the stone. The model is backed up by experimental evidence and is based on the specific tribological conditions presented by the contact between a scratched curling stone and a pebbled ice sheet.

In the sport of curling, stones are slid across an ice sheet, aimed towards a target area. A sliding stone does not move in a straight line, but follows a curled trajectory, deviating in a direction determined by its rotation. As yet, no satisfactory explanation for this motion has been presented, although many attempts have been made. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, typically such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In this paper, the motion of a rotating curling stone sliding over ice is calculated, for different assumed distributions of the coefficient of friction in the contact between stone and ice, using a numerical method. It is shown that no redistribution of the friction, no matter how extreme, can explain the observed motion of a real curling stone.

In the sport of curling, stones are slid across an ice sheet, aimed towards a target area. A sliding stone does not move in a straight line, but follows a curled trajectory, deviating in a direction determined by its rotation. As of yet, no satisfactory explanation for this motion has been presented, although many attempts have been made. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, typically such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In this paper, the motion of a rotating curling stone sliding over ice is calculated, for different assumed distributions of the coefficient of friction in the contact between stone and ice, using a numerical method. It is shown that no redistribution of the friction, no matter how extreme, can explain the observed motion of a real curling stone.

This study presents a comparative analysis of surface characteristics and properties of anodised aluminium cylinders produced by sand casting, permanent mould casting, extrusion, and high pressure die casting. Differences in micro structure and distribution of silicon particles in the aluminium, due to the fabrication method and the silicon content in the alloy, resulted in varying thickness of the oxide layers (mean thicknesses between 7 and 19 m) and surface topography. The oxide layer was unevenly thick for the permanent mould cast and the sand cast cylinders, resulting in a surface with higher plateaus and lower areas. This was more prominent for the sand cast surface. The oxide of the extruded cylinder was thick and even and its surface was smooth. The high pressure die cast surface had an oxide that was very thin and uneven. The surfaces displayed different results in the scratch test due to the variations in the surface structure. For the permanent mould cast and the sand cast surfaces the silicon particles present in the oxide deflected the cracks that were formed during the scratching. This resulted in smaller wear debris. The nanohardness values of the oxides had a large scattering due to the inhomogeneous nature of the oxide layers, with pores and particles. However, the highest nanohardness values were between 5000 and 6000 MPa for the four surfaces, which is significantly lower than that of sintered alumina. In the micro abrasion test the wear mechanism for all four surfaces was microcutting resulting in chippings.

Maintaining a reasonably low cutting tool wear when producing forming tools is a general challenge in the development of new forming tool materials. The tool life of a hot forming tool steel (H13) has been significantly improved by reducing its Si-content from 1.0 to 0.06 wt.%. However, this modified H13 (MH13) also displays a reduced cutting tool life due to higher cutting forces and a stronger tendency to form built up layers (BUE) on the cutting edge. This paper explains why. Gleeble tests of MH13 revealed a significantly higher flow stress in the 820–900 °C temperature interval in MH13 compared to H13. Thermo-Calc simulations showed that when reducing the Si-content from 1.0 to 0.06 wt.% the initial temperature for ferrite-to-austenite transformation (A1) was reduced from 900 °C to 820 °C. Knowing that austenite has totally different mechanical and thermal properties than ferrite, the difference in A1 between the two steels explains the higher cutting forces and higher tendency for BUE-formation. The conclusion is that the difference in machinability between H13 and MH13 is primarily related to their difference in A1. An attempt was also made to find a new tool material composition that can combine the wear resistance of MH13 and the good machinability of H13. Thermo-Calc simulations were performed with slightly modified alloying content without changing its properties as a good forming tool material, with the aim to increase A1. For instance, reducing the Mn content from 0.5 to 0.05 wt.% proved to increase A1 from 820 to 850 °C.

The surface properties of Fe-rich ferromagnetic Fe-Cr alloys are investigated using a first-principles quantum-mechanical method. In dilute alloys, the surfaces are dominated by Fe, whereas the Cr-containing surfaces become favorable when the bulk Cr concentration exceeds the limit of similar to 10 atomic per cent. The abrupt change in the surface behavior is the consequence of complex competing magneto-chemical interactions between the alloying atoms. Considering the quantities of various features: equilibrium surface profiles, chemical potentials, segregation energies, surface energies, magnetic moments, mixing energies and pair interactions, within a wider range of bulk and surface concentrations enables us to build a comprehensive picture of the physics of Fe-Cr surfaces. Using the present achievements many previously controversial results can now be merged into a consistent model of Fe-rich Fe-Cr alloys.

Curling is an Olympic winter sport in which 8 players forming two teams slide stones across a sheet of ice towards a target area. The two teams have eight stones each and take turns to slide the stones over to the target area, some 28 m away from the release line. After being released, the stone is only affected by the sliding friction. However, this friction may be somewhat modified by sweeping the ice just in front of the sliding stone, using special curling brooms. Further, the trajectory of the stone becomes slightly curled. By slowly turning the stone clockwise when it is released, it will turn to the right, and vice versa. The best team in each round of 16 stones score one point for each stone resting closer to the target than the best stone from the opponent team. The game makes up a very interesting tribological system, presenting a number of challenging problems. These problems include understanding exactly: what determines the level of friction, and how it is affected by the sweeping, how the roughness of the stone influences the friction, and how the sliding surface should best be prepared to give a stable and repeatable friction, how the intentionally bumpy "pebbled" ice structure influences the friction, the size and distribution of the contact spots between the rough stone and the pebbled ice, the mechanism causing the curl of the stone - "Why does the stone curl?". This poster is based on an experimental project, and presents this very intriguing tribological system. It offers some ready explanations and challenges the visiting tribologists to contribute their insights.

Influence from surface roughness of a forming tool surface on its ability to pick up work material was investigated by laboratory testing. One powder metallurgical tool steel was given different surface roughnesses and tested in sliding contact against one austenitic stainless steel (SS) and one aluminium alloy (Al). Generally, the risk of work material adhesion and transfer increased with the roughness of the tool material. However, SS gave a low friction and very little metal transfer for a tool roughness of the order of the relatively soft SS oxide thickness. The Al alloy, on the other hand, which oxide is much harder than the parent metal, has no critical surface roughness below which adhesive transfer will not occur. The oxide fractures and oxide fragments indent the tool surface increasing the risk of adhesion.

A characteristic of almost any type of dry or boundary lubricated tribological contact is that the composition and tribological properties of the original interface will change during use. The modifications include topography changes (smoothening or roughing), formation of micro-cracks, phase transformations, deformation hardening, formation of oxides, formation of solid films by reactions with lubricant additives, transfer of material from the counter surface, and so on. The thickness of these layers and films range from atomic monolayers (e.g., hydrogen termination of diamond surfaces) to tens of mu m (e.g., plastic deformation of metals). Since the tribological properties of tools, wear parts and mechanical components then are determined by these modified surfaces rather than by the original, they deserve attention and careful assessment. Without the knowledge about how these surface layers are formed and how they modify the tribosystem, it is not possible to predict the friction and wear properties of a material in a given tribological situation. Thus, post-test evaluation including microscopy and analysis of both surfaces and cross-sections is required. A number of surface modifications selected from a wide variety of tribosystems is briefly presented. The corresponding surface modifications range from the most well known and simple to the more complex and relatively unexplored. Finally, the potential of designing materials and lubricants for improved surface modification is discussed and examples are given of current research projects on promoting formation of protecting low-friction tribofilms in boundary-lubricated systems.

Pneumatic clutch actuators, employed in trucks, have been investigated. The surfaces of the inside of the anodised aluminium cylinders, of the lip seals and of the guiding rings have been studied by SEM and optical microscopy. For most of the actuators no significant wear was revealed. However, one actuator, with leakage problems, was severely worn. An extensive amount of scratches was found on the cylinder surface, the thickness of the guiding ring had decreased and large parts were missing from the lip seal. A possible explanation is that particles have entered the system thus altering the contact conditions. A test setup was developed to investigate how particles present in the system affect the wear. A part of the piston, with lip seal and guiding ring, slides against a part of the anodised aluminium cylinder in a back- and forward motion. Pure silicone grease lubricated tests resulted in no wear. Tests with added dust particles resulted in distinct scratches on the aluminium surface and embedded particles in the guiding ring and the lip seal. These tests provide results in good agreement with the wear revealed in the investigations of used actuators and support the theory that wear is caused by particles.

A hob is an advanced gear cutting tool usually made of homogenous high-speed steel (HSS) and coated by physical vapour deposition (PVD). It is designed for regrinding and recoating many times. However, hobs of today suffer from unpredictable wear making it difficult to schedule when they should be taken out for reconditioning. This investigation is aimed to contribute to the fundamental knowledge of the wear mechanisms and wear propagation of hobs used in dry gear cutting. Two AlCrN-coated hobs, used in actual gear cutting, were investigated by scanning electron microscopy (SEM) to study the tool topography, light optical microscopy (LOM) to study metallographic cross-sections and by X-ray photoelectron spectroscopy (XPS) for surface analyses. It is concluded that the high potential of PVD-coated HSS as tool material is not taken full advantage of. The main reason is a combination of a poor surface preparation prior to coating and excessively high compressive residual stresses, making parts of the coating detach along the edge line even on unused cutting teeth. During cutting, the high intrinsic stresses in combination with droplets and defects in the coating facilitated its fragmentation through decohesion and detachment. Simultaneously, the rake face is gradually covered by a thin oxide layer dominated by Fe, Cr and Mn in about equal amounts, and also with a significant content of Si. It is believed but not proved that this layer is beneficial. To improve service life and reliability of gear cutting hobs, precautions should be taken for the grinding–polishing preparation prior to PVD-coating to ensure a smooth substrate free from burning damage and other defects. Also, the edge radius and coating thickness has to be matched with the magnitude of compressive residual stresses in the coating., KUGG

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS2 coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 m TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS2 and DLC coatings, being KC = 4-11, about 2, and 1-2 MPa m1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface., Tribological performance

Versatile and reliable techniques for evaluation of hard thin coatings are necessary for the development and tribological assessment of new coatings. We have proposed a new type of micro slurry-jet erosion (MSE) test, i.e. a solid particle impact erosion test for swift evaluation of wear properties of hard thin coatings. We are using a new type of MSE test apparatus (pot type tester) that makes it possible to obtain the wear loss per unit mass of erodent, which in this test was alumina particles with an average size of 1.2 mu m. Its performance was evaluated by using a Si wafer plate under various test condition. In addition, the MSE tester was demonstrated by evaluating the wear resistance of TiN on high-speed steel substrate. The new MSE test generates highly reproducible results and is very sensitive to the quality of the coatings. Copyright (C) 2009 John Wiley & Sons, Ltd.

Several different experimental techniques are normally used to evaluate tribological properties of thin hard coatings. However, most of the tests cannot assess properties of coating, substrate and interface independently. In this study, the potential of a Micro-Slurry-jet Erosion (MSE) test to swiftly evaluate the intrinsic wear properties of thin single layered and multilayered coatings is demonstrated. A slurry containing 1.2 mu m alumina particles was impacted at high velocity perpendicular to CVD TiC/TiN (TiN on top of TiC) and Tic coatings deposited on cemented carbide. In addition, nano-indentation and XRD, GDOES, AFM analyses were done for the original and worn surfaces. For the TiC/TiN coating, the wear depth of the TiN layer increased linearly until the Tic layer was reached. There after the wear occurred at a lower rate but tended to increase towards the TiC/substrate interlayer. The single layered Tic coating showed the same wear behaviour as the TiC layer of the TiC/TiN coating. The wear rate was constant within the individual TiN and Tic layers, but changed near the interfaces. The micro-topographies of the worn surfaces observed by AFM correlated to the difference in the wear rate of the individual layers and the change in wear rate at the interface to the substrate. The Tic layer proved to have about two times higher wear resistance than the TiN layer. Consequently, the MSE test can distinguish between small differences in the properties of the coatings and of the interlayer zones, and can be used as a screening test when evaluating multilayered as well as single layered coatings.

A very attractive feature possessed by many carbon-based coatings is the ability to provide a combination of lowered friction and high wear resistance in various sliding contact situations. The flair for low friction is commonly attributed to large amounts of surplus carbon present in the microstructure. However, the surplus carbon also makes these coatings rather susceptible to oxidation. To expand the field of application for carbon-based low-friction coatings it is important to improve their oxidation resistance. Of course this must not be accomplished by impairing their main function, of giving low friction. In an attempt to increase the oxidation resistance of a TaC:C coating without sacrificing its extremely good tribological behaviour, Al was added in increasing amounts at the expense of carbon. The effects of Al addition was evaluated on as-deposited coatings from a mechanical and tribological standpoint. The oxidation resistance of the coatings was evaluated in air at temperatures up to 500C and the resulting oxides were analyzed as to their chemistry and thickness. The oxidation rate was found to decrease quite rapidly with increasing amounts of Al. Moreover, it was found that the Al addition could be optimized to give low friction and reduced oxidation rate at only a moderate increase in wear rate., Wear rates

Austenitic stainless steels are known to be very difficult to machine and form. This is due to their tendency to adhere to the tool material, which then initiates galling. In this paper, austenitic stainless steel has been tested against TiN in un-lubricated sliding contact. The interface between TiN and adhered stainless steel was investigated by transmission electron microscopy. An oxide layer, emanating from the stainless steel, was revealed to be present between the tool surface and the adhered stainless steel. It is concluded that the stainless steel oxide plays a major role for the sticky behavior of this material. To further investigate this phenomenon, austenitic stainless steel samples were also pre-oxidised before testing to examine if the oxide thickness and composition are important for the adhesion tendency. The test showed that the sample oxidised at 800 °C had less tendency to adhere than those oxidised at lower temperatures. The explanation is either its higher thickness, or its high content of Cr-oxide in its surface. The surface of the oxides formed at lower temperatures consists mainly of iron oxide.

The transfer of work material to the tool surface limits the tool life in many forming operations. Using a dedicated load-scanning test equipment with crossed-cylinder geometry, dry forming of austenitic stainless steel was simulated by provoking adhesion to the TiN-coated tool specimen. High-resolution electron microscopy combined with analytical techniques was used to examine the interface between tool and work material. The decisive mechanism for adhesion and transfer of steel to the TiN surface is suggested. The oxide layer on the steel surface, especially the Fe-oxide, initiates the metal transfer. The interfacial oxide acts as a glue between stainless steel and TiN and increases the adhesive forces. Obviously, the adhesion and internal strength of the oxide layer is far stronger than anticipated. It may even be stronger than the bonding to the austenitic steel itself. A consequence of these findings is that the development of galling resistance-forming tool materials and coatings for austenitic stainless steels should not only aim to improve the bulk tool material, but also to reduce the adhesion strength between the tool surface and the oxide layer on the work material.

Most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. However, theoretical predictions of the hardness of this kind of materials have rarely been addressed in literature. In this paper, a hardness formula for multicomponent covalent solid solution is proposed based on the work of Simunek and Vackar [Phys. Rev. Lett. 96, 085501 (2006)]. With this formula, the composition dependence of the hardness is investigated for titanium nitrogencarbide (TiN1-xCx), off-stoichiometric transition-metal nitrides (TiN1-x and VN1-x), and B-doped semiconductors. The predicted hardness is in good agreement with experiments. To investigate the most frequently quoted correlation between hardness and elastic modulus, the elastic moduli of the systems involved in this paper have also been calculated. The results show that the elastic moduli cannot be used for rigorous predictions of the hardness of the solid solutions.

In percussive and rotary percussive rock drilling, the rock is crushed into small fragments by the repeated hard impact of the drill bit, and subsequently removed by flushing water or air. To avoid excessive wear, the steel drill bit is equipped with a set of cemented carbide buttons that protrude from the bit to take the actual impact. The severe contact against the rock results in some wear of the button, but also in formation of surface layers of rock material and penetration and impregnation of rock material into the cemented carbide structure. This situation, with serious implications for the wear and fracture of the buttons, have previously not been reported. The present findings represent a significantly new understanding of the wear of the rock button material. The deterioration mechanisms are described in detail, using examples from a range of real drilling applications in different rock types. During operation, material in the surface layer of the drill button shifts from that of the original cemented carbide into an uncontrolled composite. This composite is formed by the WC carbide hard phase and a binder consisting of a mixture of cobalt and rock. This new material should be expected to exhibit properties significantly different from the original cemented carbide.

Electrical discharge machining (EDM) is a non-traditional machining method extensively used to manufacture complex geometries of hard and brittle materials such as WC-Co cemented carbides (CC). Although the thermal action of the EDM process is known to yield a relatively poor surface integrity in these materials, it may be minimized through the implementation of multi-step sequential EDM and post-EDM surface treatments. Particularly, hard coating application has been demonstrated to be effective for decreasing the EDM-induced mechanical degradation. However, additional studies are required on such coating-EDMed substrate systems to determine other crucial properties in terms of applications, e.g. adhesion and micro-scale wear behaviour. In this work the adhesion strength and the microabrasive wear resistance of TiN deposited on EDMed substrates have been evaluated by means of scratch and crater grinder testing, respectively. The results indicate that both critical load for decohesion of the coating from the substrate and coating specific wear rate increase with finer-executed EDM, reaching values close to those measured for a TiN coating deposited on a ground and polished substrate.

Influence from surface roughness of a forming tool surface on its ability to pick up work material was investigated by laboratory testing. One powder metallurgical tool steel was given different surface roughnesses and tested in sliding contact against one austenitic stainless steel (SS) and one aluminium alloy (Al). Generally, the risk of work material adhesion and transfer increased with the roughness of the tool material. However, SS gave a low friction and very little metal transfer for a tool roughness of the order of the relatively soft SS oxide thickness. The Al alloy, on the other hand, which oxide is much harder than the parent metal, has no critical surface roughness below which adhesive transfer will not occur. The oxide fractures and oxide fragments indent the tool surface increasing the risk of adhesion.

We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A=C,Si,Ge) and applied to titanium nitrogen carbide (TiN1-xCx with 0<=x<=1), off-stoichiometric transition-metal nitride (TiN1-x and VN1-x with x<=0.25), and B-doped semiconductors (C1-xBx, Si1-xBx, and Ge1-xBx with x<=0.1). In all cases, the theoretical hardness is in good agreement with experiments.

Usingfirst-principles quantum-mechanical theory, we demonstrate that the surface chemistry ofFe-Cr alloys follows the peculiar threshold behavior characteristic of ferriticstainless steels. We find that in dilute alloys the surfacesare covered exclusively by Fe, whereas for bulk Cr concentrationabove ~10% the Cr-containing surfaces become favorable. The two distinctlydissimilar surface regimes appear as a consequence of two competingmagnetic effects: the magnetically induced immiscibility in bulk Fe-Cr alloysand the stability of magnetic surfaces.

Two test methods often used to simulate the tribological performance of forming tool materials are compared in this investigation through an evaluation of the friction and wear properties of four tool steels in dry sliding. One test (slider-on-flat-surface (SOFS)) utilises a vertical disc sliding on a horizontal flat test surface, and the other (load scanner (LS) tests) utilises two crossed cylindrical rods. The test conditions were selected as equal as possible for the two tests, and the following conclusions are made.Somewhat unexpectedly, the friction and wear results differed substantially between the two tests. The SOFS test showed a better potential to evaluate wear resistance since one test sample is in continuous contact with the other. The LS test can generate higher contact pressures since the two rods contact each other in an unworn condition throughout the whole test stroke.LS indicate that two hard and smooth tool steels tested against each other generally give low friction and good galling resistance. The two hard couples tested sustained the highest loads without any detectable surface damage. For the same combinations of hard steels, SOFS gave a higher friction due to the wear of the disc. The carbides in the disc material resist wear better than the matrix and will consequently wear the disc by abrasion, which adds to the friction.The above conclusions are drawn from a rather limited examination using only one set of test parameters. In fact, the two tests are both very flexible as to the way they can be used, and they both have advantages and limitations in tribological studies. They should rather be considered complementary than competitive.

This paper is focused to the metallurgical consequences of severe adhesive wear of metallicmaterials. Early examples from the late 70:ies from sliding wear tests of different steels areshown together with some high-resolution TEM micrographs of a cemented carbide cutting tooledge, prepared by using a Focused Ion Beam.Irrespective of sliding conditions, severe metallic wear of the adhesive type results in a surfacelayer, the structure of which is totally different from that of the original bulk material. Theoutermost surface layer displays a nano-crystalline structure followed by a textured layer inwhich the original grains are heavily deformed. For carbon steels, the nano-crystalline layeroften represents untempered martensite.During the wear process, oxide fragments and wear particles from the counter-material may alsobe mixed into the surface layer.The consequence for all metallic materials is that severe wear generates a hard superficial layer.For carbon steels, the hardness of the outermost layer may well exceed 1000 HV. The hardeningmechanisms are well known to a metallurgist and consist of grain refinement, deformationhardening through dislocation generation and tangling, solute hardening (martensite in carbonsteels) and second phase or particle strengthening through intermixing.Consequently, the wear process generates a surface layer on metallic materials that has a muchhigher wear resistance than the original material. This was also demonstrated in one of theexperiments.

The aim of the present work was to investigate and compare different surface engineering techniques in terms of galling properties of forming tool steel when sliding against different work materials. The surface engineering techniques included were different grades of polishing, plasma nitriding and different hard coatings, all applied to cold work tool steel respectively. Investigation results indicate that the galling tendency can be greatly reduced by proper polishing of the tool surface. If polished after treatment, plasma nitrided surface will provide further improvement in friction and wear properties of the tool steel and especially for aluminium and titanium alloys reduce the galling tendency. On the other hand, coating selection for improved galling performance of coated forming tool steel greatly depends on the type of work material. In the case of stainless steel, carbon based coatings provide the best protection against the work material transfer, while forming of aluminium and titanium based alloys, requires nitride type coatings.

Ceramic carbon based coatings have become common on tools and machine components, mainly because they offer a greatly reduced friction, as compared to uncoated steel contacts, in combination with a considerable wear protection. Very often they contain alloying elements such as different metals to control properties like hardness, wear resistance and lubricity. Applied to component surfaces, also properties relating to the running-in behaviour become important since an efficient running-in is crucial for their long term steady-state functionality. Also, even though component coatings are meant to provide low friction in dry sliding, they are increasingly used also in lubricated situations. This raises demands for compatibility with common lubricants. All these aspects have to be considered when developing new tribological coatings and some of these are addressed in this work. Results from different sliding tests with coated and uncoated surfaces in both dry and boundary lubricated contacts are presented. The generated tribofilms and their influence on the tribological properties are compared for the different sliding conditions. It is also shown how lubricants and their additives can affect the running-in, the tribofilm formation, and alter the steady state friction.