N.J.M. Carvalho

Investigation on the formation of tungsten carbide in tungsten-containing diamond like carbon coatings

A series of tungsten-containing diamond-like carbon (Me-DLC) coatings have been produced by unbalanced magnetron sputtering using a Hauzer HTC-1000 production PVD system. Sputtering from WC targets has been used to form W-C:H coatings. The metal to carbon ratio has been varied to study changes in the metal carbide formation and distribution within the amorphous hydrocarbons (a-C:H) matrix. The difference in the formation of the metal carbide is then linked to changes in the mechanical and tribological properties of the coatings. Detailed high-resolution cross-section TEM has been carried out to analyze the microstructure of the coatings. By changing the amount of a-C:H in the W-C:H coatings, the coefficient of friction could be varied between 0.129 and 0.312. The hardness was found to vary between 8 and 27.5 GPa by using different acetylene gas flows. It was observed that all coatings did have a pronounced multilayered structure.

Properties and characterization of multilayers of carbides and diamond-like carbon

Abstract Metal containing diamond-like carbon (Me-DLC) coatings are widely applied in industrial applications. Normally, the coatings are produced with small inclusions of carbide forming elements like the 3d, 4d or 5d metals, or Si or B. The small carbide islands have sizes of approximately 2–20 nm. The effect of the nano inclusions is a reduction of internal compressive stress, a lowering of Youngs modulus, a lower hardness, and a higher coefficient of friction as compared to pure diamond-like carbon (DLC). The contents of metallic elements are typically between 10 and 20 at.%; for B containing diamond-like carbon a higher percentage is observed (up to 50% B). In this work the effect of replacing the nano inclusions by a stack of extremely thin carbide layers, separated by carbon layers, is tested. Properties like adhesion, hardness, E-modulus, fatigue resistance, coefficient of friction and wear resistance were studied. Furthermore, detailed high resolution TEM was performed to observe the effects of layer integrity. The results show a clear difference in wear and fatigue behavior when the multilayer structure was altered. Short periods within the multilayer structure of W-C:H promotes the wear resistance, while long periods promote the fatigue resistance.

Microstructure investigation of magnetron sputtered WC/C coatings deposited on steel substrates

Abstract Electron microscopy, including scanning (SEM), transmission (TEM) and high-resolution (HRTEM) were employed to characterise slightly different tungsten carbide/carbon coatings deposited onto steel substrates. Complementary techniques, such as X-ray diffraction (XRD), Auger electron spectroscopy (AES) and energy filtered TEM (GIF) were also used. The coatings were deposited by magnetron sputtering of pure WC and Cr targets in a plasma decomposition of C 2 H 2 in mixed Ar–C 2 H 2 discharges. The coatings are made up of a chromium interlayer, a coarse WC/C intermultilayer, a WC layer, and the WC/C multilayer. The chromium interlayer has a body centred cubic phase and a dense columnar structure, while the remaining coating is truly amorphous, with the exception of polycrystalline particles and clusters that are present within some layers. Crystalline particles and clusters were identified as having the cubic β-WC 1− x phase. Defects in the coatings were also found, due to substrate surface irregularities and to the growth structure of the chromium columns.

Evolution of the microstructure, residual stresses, and mechanical properties of W-Si-N coatings after thermal annealing

W-Si-N films were deposited by reactive sputtering in a Ar + N-2 atmosphere from a W target encrusted with different number of Si pieces and followed by a thermal annealing at increasing temperatures up to 900 degrees C. Three iron-based substrates with different thermal expansion coefficients, in the range of 1.5 x 10(-6) to 18 x 10(-6) K-1 were used. The chemical composition, structure, residual stress, hardness (H), and Youngs modulus (E) were evaluated after all the annealing steps. The as-deposited film with low N and Si contents was crystalline whereas the one with higher contents was amorphous. After thermal annealing at 900 degrees C the amorphous film crystallized as body-centered cubic alpha-W. The crystalline as-deposited film presented the same phase even after annealing. There were no significant changes in the properties of both films up to 800 degrees C annealing. However, at 900 degrees C, a strong decrease and increase in the hardness were observed for the crystalline and amorphous films, respectively. It was possible to find a good correlation between the residual stress and the hardness of the films. In several cases, particularly for the amorphous coating, H/E higher than 0.1 was reached, which envisages good tribological behavior. The two methods (curvature and x-ray diffraction) used for calculation of the residual stress of the coatings showed fairly good agreement in the results.

Stress state of TiN/TiAlN PVD multilayers

Abstract A multilayer system consisting of TiN and TiAlN layers is deposited by means of a PVD process onto stainless and tool steel substrates. The study is aimed at determining the microstructure and the macrostresses in these layers with X-ray diffraction. The multilayer system is composed of a relative thick TiAlN layer (∼150 nm) and a set of smaller alternating TiN/TiAlN layers of approximately 15–20 nm each with a total thickness of 150 nm. This basic building block of the structure is repeated throughout the coating and is sandwiched between two thicker layers: a TiN layer (400 nm) to achieve good adhesion with the substrate, and a top layer of TiAlN (400 nm). The total thickness of the coating is approximately 4·4 μm. From X-ray diffraction it is concluded that the layers are only slightly textured and there is a weak (311) texture. The strain measurements show a difference in strain for the layers on stainless and tool steels, which is owing to a difference in the linear expansion coefficient for the two substrates. It is possible to determine the unstrained lattice spacing of the TiN and the TiAlN sublayers and to calculate Poisson’s ratio for both materials. Furthermore, the residual stresses in the different sublayers could be derived and it was found that they were much higher in the TiN than in the TiAlN. This may be explained by the thermal origin of the residual stress in the TiAlN sublayers, whereas in the TiN sublayers the atomic peening process during deposition introduces an additional residual stress.

Interfacial fatigue stress in PVD TiN coated tool steels under rolling contact fatigue conditions

Titanium-nitrogen (TiN) films were Physical Vapour Deposited (PVD) on tool steel substrates with different hardness and surface roughness, in a Bai 640R unit using a triode ion plating (e-gun) with a high plasma density. The coated substrates were submitted to a rolling contact fatigue test technique (modified pin-on-ring test) to obtain some clarifications of the mechanism of interfacial failure. Tests were run using PVD-coated rings finished by polishing or grinding to produce different surface roughnesses. From the results, it appears that the fatigue durability is at lower stress levels significantly influenced by both the pre-treatment and the final surface roughness of the material. The polished and smoother surfaces are associated with a longer fatigue life. However, at a higher contact stress, there appears to be very little influence of pre-treatment and surface roughness. Two mechanisms of crack propagation under pure rolling conditions were found, depending on the substrate hardness. For the softer substrates, the cracks propagate mainly perpendicular to the surface, whereas for the harder substrate, the cracks generally originate at the interface and progress in the coating parallel to the surface.

Nanoindentation study of PVD WC/C coatings supported by cross- sectional electron microscopy observations

Abstract Coatings consisting of multilayer tungsten carbide/carbon (WC/C) were deposited by physical vapour deposition (PVD) onto steel substrates. The goal of this investigation was to probe the outcome of coatings dissimilarities as well as the substrate employed on the mechanical properties of the system by nanoindentation techniques. The coatings investigated are mainly composed of a chromium columnar interlayer to improve the adhesion to the substrate, an inter-multilayer of WC and carbon, and an interlaminar amorphous WC/C multilayer structure. The mechanical properties were assessed by nanoindentation techniques. The hysteresis loops were analysed and discussed in terms of the method developed by Oliver and Pharr. Moreover, a new technique for cross-sectional transmission electron microscopy of the nanoindentations has been developed, and the information obtained was correlated with the load–displacement data from a nanoindentation cycle to clarify the extent of any delamination fracture.

On the microstructure and deformation mechanisms under indentations of TiN/(Ti,Al)N multilayer coatings

Commercially available multilayer TiN/(Ti,AI)N coatings deposited onto stainless steel substrates have been evaluated with respect to their microstructure and deformation mechanisms. The microstructure and chemical composition have been investigated using a combination of scanning electron microscopy cross-sectional transmission electron microscopy and electron energy loss spectroscopy. The multilayer exhibited a columnar structure extending throughout the film thickness. Micrometer-sized macroparticles were present at various distances from the substrate, being incorporated in the growing film in the solid state. They consisted of a core structure with equiaxed grains having the alpha-Ti phase and an outer layer of TiN. Nanoindentation testing was used to explore the deformation mechanisms operating in the coated system. Analysis of the load-displacement curves showed that they were useful in identifying the occurrence of cracking. By cross-sectioning the nanoindentations, the deformation was observed to occur along the columnar grain boundaries and in the layers parallel to the interfaces. The effect of the macroparticles in the deformation mechanisms has been observed and will be discussed.

Microstructure of WC/C coatings deposited on steel substrates

Electron microscopy, including scanning (SEM), transmission (TEM) and high-resolution (HRTEM) were employed to characterise slightly different tungsten carbide/carbon coatings deposited onto steel substrates. Complementary techniques, such as X-ray diffraction (XRD), Auger electron spectroscopy (AES) and energy filtered TEM (GIF) were also used. The coatings were deposited by magnetron sputtering of pure WC and Cr targets in a plasma decomposition of C2H2 in mixed Ar-C2H2 discharges. The coatings are made up of a chromium interlayer, a coarse WC/C intermultilayer, a WC layer, and the WC/C multilayer. The chromium interlayer has a body centred cubic phase and a dense columnar structure, while the remaining coating is truly amorphous, with the exception of polycrystalline particles and clusters that are present within some layers. Crystalline particles and clusters were identified as having the cubic beta -WC1-x phase. Defects in the coatings were also found, due to substrate surface irregularities and to the growth structure of the chromium columns.

Nano Indentations Studies of WC/C and TiN/(Ti,Al)N Multilayer PVD Coatings Combined with Cross-sectional Electron Microscopy Observations

Multilayers of tungsten carbide/carbon (WC/C) with an amorphous structure and multilayers of titanium nitride/titanium-aluminum nitride (TiN/(Ti,Al)N) with a polycrystalline structure, prepared by physical vapor deposition, have been subjected to nanoindentation testing. The investigation has been aimed at establishing whether the load-displacement responses provides accurate information on the fracture mechanisms and whether such mechanisms can be characterized using a new technique for cross-sectional electron microscopy of the nanoindentations. Analysis of the load-displacement curves showed that they can be used to identify the cracking mechanisms occurring in the multilayers and that cross-sectioning of the nanoindentations is necessary if a more complete understanding of the multilayer coatings behavior is required.