Per Hedenqvist

ME-C:H coatings in motor vehicles

Abstract The trends in the automotive industry are towards higher performance, improved reliability and tolerances, less lubricants and more environmental friendly products. In addition, there is a strong demand for decreased fuel consumption. All this is a tough challenge for the automotive designers who will need new ways of thinking and occasionally unconventional solutions. A solution is to improve the wear resistance and lower the friction of the tribological system by deposition of a low friction diamond like carbon (DLC) coating on critical components. Me-C:H is a DLC type of coating which has been tested on a large number of automotive components during the last 10 years. Today, parts coated in serial production are mainly to be found in diesel injection systems. However, the use of Me-C:H coatings in vehicles increases rapidly. Nearly 30 million coated parts are delivered to the automotive industry every year with an annual increase of approximately 50%. In this paper, an overview of results from laboratory testing as well as real applications are presented and discussed. Emphasis will be put on the aspects of design to optimise the tribological system; e.g. what is required of the substrate to ensure a successful use of the coated component. The majority of the results are positive; friction has decreased drastically and lifetime, wear resistance and resistance to seizure has increased significantly. The high performance of Me-C:H coatings and the high potential of an optimised tribological system predict a further rapid growth of these coatings in the automotive industry.

A new test method for the intrinsic abrasion resistance of thin coatings

Abstract A new method has been developed for checking the mechanical quality of thin coatings by determining their resistance to small-scale abrasion. The method is capable of determining the wear constants for the coating and the substrate individually, even for very thin coatings. The theory for imposed shape wear scars is introduced. Suitable parametersaand test procedures are proposed and the repeatability, experimental scatter and general capability of the method are presented. The method has the following advantages: 1. (1) The intrinsic abrasion resistance of the coating and the substrate are measured individually. In contrast to competing wear tests which evaluate the wear resistance of the composite rather than the coating and the substrate individually, these values are independent of the wear scar size. 2. (2) Very thin coatings can be evaluated. In the most favourable situation, where the coating has a considerably higher abrasion resistance than the substrate, the wear constant of coatings of sub-micron thickness can be readily determined. 3. (3) The small volume needed makes the test virtually non-destructive. For example it is fully possible to test the coating of a cutting insert before using it in a tool life test. 4. (4) The craters are produced in a commercially available dimple grinder, which remains useful for its original purposes of preparing transmission electron microscopy samples and coating thickness measurements.

Failure mode analysis of TiN-coated high speed steel: In situ scratch adhesion testing in the scanning electron microscope

Abstract A scratch test apparatus for in situ testing of coating adhesion in the scanning electron microscope has been designed. Scratch tests were performed on TiN-coated high speed steel substrates with various coating thicknesses and substrate hardnesses. Four groups of coating damage and detachment mechanisms were identified: deformation, crack formation, chip formation and flaking. In all cases, incipient coating failure was associated with a sharp increase in the friction force readings. The maximum normal force that the coating-substrate composite could sustain increased significantly with increasing coating thickness and substrate hardness. An interesting observation was that direct adhesive failure by interfacial fracture only occurred for the combination of a thick (3 μm) coating on a hard (1000 HV) substrate. In all other cases, coating failure was due to a ductile chip formation mechanism. The detailed influence of the coating thickness and substrate hardness on the resulting coating failure modes is illustrated in the paper by the introduction of coating failure maps. The results demonstrate the great potential for in situ studies in order to obtain a better understanding of the basic coating failure mechanisms. In situ scratching allows the dynamics of the process to be studied; the coating damage and detachment mechanisms in front of the tip can be identified and the scratching events can be directly correlated to the corresponding friction force characteristics. Unfortunately, direct extrapolation of results from the in situ test to the conventional scratch test is difficult owing to the smaller tip radius (25 instead of 200 μm) used in the in situ experiments.

Multilayer cracking resistance in bending

Abstract Degradation and failure of thin, hard coatings in tribological applications is often governed by crack initiation and growth. For this reason, cracking resistance is one of the most important coating materials properties. Multilayer coating structures are commonly assumed to have cracking resistances superior to those of conventional single layer coatings. Several studies support this assumption, but until today no reliable method that is capable of quantitatively comparing the cracking resistances of single and multilayer coatings has been presented. This work utilises a new experimental method for determination of cracking resistances of tribological vapour deposited coatings. Coated beams of high speed steel are subjected to four point bending while simultaneously detecting the applied load and the acoustic emission resulting from crack formation and growth. As the bending device can be inserted in a scanning electron microscope (SEM), it is also possible to observe the beams during bending. The SEM allows the high magnifications necessary to reveal cracks and observe subtle differences in cracking behaviour between, e.g. single layer and multilayer coatings. Several single layer and multilayer coatings, consisting of various combinations of evaporated Ti and TiN and sputtered NbN and TaN, are evaluated. The multilayer coatings all contain a large number of lamellae and are produced by repeated, alternating deposition of two of the materials. The different types of coatings are found to behave quite differently during the bending procedure, and it is shown that the multilayer coatings have indeed higher cracking resistance than the single layer coatings.

How TiN coatings improve the performance of high speed steel cutting tools.

Abstract TiN coatings deposited onto high speed steel (HSS) by physical vapour deposition have successfully been used in metal cutting applications for a number of years. Many papers have been presented which all show that TiN coatings often yield decreased wear rates and, usually, reduced friction coefficients. Although different theories are proposed in the literature (hardness theory, diffusion barrier theory, thermal barrier theory, reduced friction theory etc.), most papers have not dealt with the question of how and why TiN coatings modify the performance of HSS cutting tools. The present paper contributes to the answer of this question by (1) organizing published information about significant mechanical and chemical properties of TiN and (2) closely discussing different proposed theories and making comparisons with results from the broad spectrum of experiments (tool wear simulation testing, scratch testing in a scanning electron microscope, particle erosion testing, chip formation studies, metal cutting etc.) performed by the authors. A unique combination of sufficient adhesion to the substrate, high hot hardness, high wear resistance and an ability to improve the contact conditions at the cutting edge is concluded to be the answer. The complex interrelationships encompassed by this explanation are displayed in a flowchart. The thermal barrier function and the relatively low sliding friction coefficient of TiN are believed to be of minor importance.

Cracking resistance of thin hard coatings estimated by four-point bending

In this work, a fast and easily performed four-point bending test for evaluation of cracking resistance of thin hard coatings is presented. A bending device, small enough to be put in an SEM and observed in situ, has been designed. By crack formation studies, a measure of coating cracking resistance is obtained. Two methods of crack detection are utilised: detection of acoustic emission and direct observation in the SEM. The two methods yield virtually identical values of the cracking resistance. However, the acoustic evaluation is much faster and easier to perform and is therefore to be preferred in long test series. SEM observations, on the other hand, allow a more straightforward interpretation. In this paper the test is used to determine the coating strain corresponding to crack initiation in TiN and CrN coatings on high speed steel. The test yields values of cracking resistance for the coatings in the range 0.1% (TiN) to 0.7% (CrN). In the SEM studies, cracks were found to nucleate predominantly at defect sites in the coating and propagate, highly aligned, perpendicular to the length of the beam. The cracks usually terminated when reaching the substrate. However, if hitting carbide at the substrate-coating interface, the crack also continues through the carbide, and terminates when reaching the substrate matrix.

Wear of PVD Ti/TiN multilayer coatings

The wear characteristics of PVD Ti/TiN multilayer coatings subjected to two-body abrasion and particle erosion have been studied using diamond slurry and silicon carbide particles as abrasive medium and erodant, respectively. The abrasive wear rate of the Ti/TiN multilayer coatings was found to increase with the relative amount of metallic Ti in the coatings. In erosion, the lowest wear rate was recorded for the homogeneous TiN coating. For the Ti/TiN multilayer coatings the erosion rate was found to decrease with an increasing relative amount of metallic Ti in the coatings. It is concluded that the concept of multilayered coatings offers a potent means to tailor the properties of tribological coatings. In particular, demands of different applications can be met by adjusting the relative thickness of metallic Ti in Ti/TiN coatings. The amount of metallic Ti can, for example, be used to control the coating residual stress state. Multilayered Ti/TiN coatings seem promising for combined wear and corrosion protection.

Deposition and mechanical properties of multilayered PVD Ti−TiN coatings

This work aims at the development of multilayered coatings with increased fracture resistance, retained hardness and adhesion (compared with current state-of-the-art coatings) to the substrate. One ...

Direct current bias applied to hot flame diamond deposition produces smooth low friction coatings

Abstract As-deposited diamond coatings generally have a high surface roughness which results in a high friction coefficient and extensive wear of the counter material in sliding contact. Therefore several methods for smoothening diamond coatings have been proposed, such as laser polishing, molten metal etching, thermochemical polishing and mechanical polishing. All these methods have some disadvantage e.g. long processing time or high processing temperature. Furthermore, they are all post-deposition treatments i.e. the manufacture of these coatings requires at least two processing steps, deposition and smoothening. With the present method which combines d.c. bias with hot flame diamond deposition, a smooth diamond surface is produced during the actual growth of the film. No post-deposition treatment is necessary. The surface roughness is not dependent on the coating thickness which means that thick coatings with smooth surface can be produced. In fact, the method has a smoothening effect, i.e. rough surfaces can be made smooth. The method is comparable to conventional hot flame deposition of diamond as to growth rate and cost of producing the coatings. The coatings have a nano-crystalline structure and a surface roughness of Ra = 25 nm, and result in a friction coefficient of 0.1 or less in dry sliding and about 0.05 in water-lubricated sliding against cemented carbide. Their wear resistance is virtually the same as that of conventional diamond films.

Sliding wear testing of coated cutting tool materials

Abstract The sliding wear characteristics of TiC-coated cemented carbide and TiN-coated high-speed steel during simulated machining conditions have been investigated using a modified pin-on-ring test. The influence of sliding distance and sliding speed on the amount of wear has been determined and the dominant wear mechanisms have been characterized using light optical microscopy and scanning electron microscopy. The sliding wear resistance of the investigated material combinations was found to be strongly dependent on the sliding speed. The dominant wear mechanisms were identified as adhesive wear (coating) and solution wear (substrate) for the TiC-coated cemented carbide while the TiN-coated high-speed steel was worn by a combination of cracking and plucking of TiN fragments and adhesive wear of the high-speed steel.