Urban Wiklund

Influence of residual stresses on fracture and delamination of thin hard coatings

The risk of fracture and delamination of residually stressed coating systems is examined. Stress concentrations are generated at the interface of coated systems where the substrate deviates from being perfectly smooth, flat and infinitely large. Using finite element calculations, such stresses induced at pores, edges or scratches are analysed for a number of representative coating/substrate systems. The effect of the interface topography, coating thickness and elastic mismatch on the interfacial stresses are investigated. Generally, thin coatings compared to the interface topography are less sensitive to residual stress induced failure. At a critical coating thickness, normal stress across the interface of a magnitude comparable to that of the residual stress level is induced, which may initiate coating delamination. The interface stress state becomes independent of the coating thickness if the coating is thicker than about three times the amplitude of the interface roughness. The interface stress scales approximately linearly with the maximum inclination of the surface profile. It is demonstrated experimentally that the high residual stress in ceramic coatings may cause local coating fracture and delamination at, e.g., the tip of an edge or at rough substrate surfaces when the coating is thick relative to the edge radius or the surface topography, respectively.

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.

An evaluation of plasma sprayed ceramic coatings by erosion, abrasion and bend testing

Abstract The mechanical properties of a series of plasma sprayed ceramic coatings made from alumina, chromia and alumina–titania precursor powders have been evaluated in particle erosion, two-body abrasion and bend testing. For the alumina coatings, both monocrystalline (sapphire) and polycrystalline alumina powders of varied grit sizes were used. Also, coatings of chromia and a mixture of alumina and titania were evaluated. The coatings were deposited with an axial injection spraying technique. Clear differences were found between the investigated coatings. Among the alumina coatings, those deposited from sapphire powders gave the best results in the wear tests. An addition of titania to alumina had a favourable effect in most tests. In abrasion, one chromia coating displayed a wear resistance comparable to the dense sintered alumina reference. The erosion rates were, however, several times higher for all sprayed coatings compared to the reference alumina.

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.

Cohesion in plasma-sprayed coatings - A comparison between evaluation methods

Abstract The integrity, and especially, the cohesive strength of alumina plasma-sprayed coatings prepared from a range of precursor powders of different size and crystallinity has been investigated by several different methods. Several chromia and alumina-titania coatings, as well as a sintered bulk alumina, have also been studied for comparison and reference purposes. Methods of characterisation have included assessment of the coatings resistance to open tribo-system wear by dry particle erosion and abrasion on the one hand, and their resistance to mechanical failure during controlled scratch testing and tensile fracture in four-point bending, on the other. Controlled scratching was carried out on both top- and cross-section, with the cross-sectional scratching done inside a scanning electron microscopy (SEM). Bending was carried out, in ambient conditions, using test equipment small enough to be placed in the SEM, in order to observe the progressive cracking behavior and failure of the coatings. Some correspondence was found between most of the techniques/methods studied, but erosion and abrasion with large, hard particles and scratch testing on both top surfaces and cross-sections provided the best correlation. It is concluded that these are the most promising methods found so far to rank the cohesion characteristics of lasma sprayed coatings.

The multilayer effect in abrasion — optimising the combination of hard and tough phases

In the present investigation it is shown that the wear resistance of multilayer PVD coatings, combining relatively soft but tough chromium (Cr) and harder but more brittle chromium nitride (CrN): exceeds that of both its phases. This striking exception to

Corrosion behaviour of Ti/TiN multilayer coated tool steel

The corrosion behaviour of Ti/TiN multilayer coated tool steel has been investigated by potentiodynamic measurements in 0.1 M H2SO4 and compared with single layer coatings of Ti and TiN, respectively. All the coatings had a total thickness of about 1 mu

Determination of growth-induced strain and thermo-elastic properties of coatings by curvature measurements

Determination of growth-induced strain and thermoelastic properties of coatings by curvature measurements

Mapping of mechanical properties of WC–Co using nanoindentation

High-resolution measurements of mechanical properties are of immense importance in metallurgy. Measuring the intrinsic properties of each phase separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. In this work, nanoindentation has been used to reveal mechanical properties of different phases in WC–Co with very high resolution. The resolution limits of the equipment and this material as well as various techniques to accomplish the measurements were evaluated. By making indents only 0.1 µm apart and using very low loads (1 mN) it was possible to distinguish between the two different phases, WC and Co. Maps created from the measured properties, hardness and Young’s modulus, were in excellent agreement with SEM images of the same area. Furthermore, it was also possible to detect an increased hardness of the Co binder phase by a factor of four as compared to the bulk hardness of Co. This work verifies experimentally what several authors have proposed earlier based on modelling.

Evaluation of new multilayered physical vapour deposition coatings in sliding contact

Abstract Much of the current development in surface engineering is focused on multilayered coatings, i.e., layered combinations of two or more materials, often metals, nitrides or carbides. This concept has proven to offer both a high hardness and an improved toughness as compared to single layer coatings. Consequently, multilayered coatings has the potential to improve the tribological properties of tools and components. Friction and wear properties of four new hard physical vapour deposition (PVD) multilayer coatings (TiN/CrN, TiN/MoN, TiN/NbN, and TiN/TaN) were studied using a ball-on-disc tester. A single-layer TiN coating was included as a reference. Three tests with different counter materials were performed. The overall best performance was found for TiN/NbN when tested against a steel ball and TiN/MoN proved to be superior against an alumina ball. Finally, as the coatings were tested in a self-mated contact against a coated ball, TiN/NbN and TiN proved to yield the best combination of low friction and low wear.