ICMCTF2015 Session B4-2: Properties and Characterization of Hard Coatings and Surfaces

Coatings are well established to solve tribological problems in automotive applications. In systems under high load and / or high pressure carbon-based coatings, usually called DLC for diamond-like carbon, reduce friction and wear. Other coatings, e. g. chromium nitride coatings, are useful to prevent scuffing.

A real challenge in evaluating coatings for their suitability in automotive applications is the broad range of conditions which defines the load collective of the tribological system. Current investigations are focused on the temperature. The temperature range of a tribological system in a car engine may be much broader than the -20°C to 120°C as given by the average oil temperature. In the frictional contact with and without temporary dry running the temperatures may be higher by up to several hundred degrees Celsius.

These local temperatures affect the properties of the elements of the tribological system; but the temperature itself may be influenced by the properties of these elements. The relatively low thermal conductivity of coatings results in high local and transient temperature peaks. This may lead to a degradation of a carbon-based coating or to a lower viscosity of an intermediate medium like oil.

A simple calculation following Blok’s approach gives an approximation for the peak temperature in the frictional contact. An uncoated surface gets a localized additional temperature share of about 125°C at 400 MPa Hertzian pressure. Due to the low thermal conductivity of some actual coatings which were measured to be in the range of 0.8 WK^-1m^-1 to 7.0 WK^-1m^-1 the respective temperature rise can be as high as 300°C.

Assuming a base temperature of 100°C this additional temperature would be well above the graphitization temperature of a DLC coating of about 350°C. While graphitization limits the application of DLC, it also allows the great running-in properties of carbon-based coatings. Measurements with a pin-on-disc test set-up and a SRV test set-up (reciprocating wear test) give a deeper insight.

It is widely accepted that coatings show a significant effect when the tribological system runs in mixed lubrication or even in boundary lubrication: body and counter-body are at least partially in contact and the friction itself is dominated by the properties of the coating. An impressive friction reduction can also be obtained under full lubrication, when body and counter-body are well separated. In this case, the relatively low thermal conductivity of coatings results in a locally reduced viscosity of the intermediate medium by the local “heat confinement”. A simple tribometer shows a friction reduction under full lubrication of up to 50%.

Thin ceramic nitride coatings such as TiN and BN deposited by PVD on steel substrates have been shown to reduce hydrogen permeability and such coating materials can be applied to the structural metals in fusion plants or the components in fuel cells. However, there was limited information on hydrogen permeation of surface-nitride stainless steel.

The hydrogen permeation behavior of surface-nitride SUS316L stainless steel was investigated in comparison with nitride coatings. Various nitride methods were applied and the relation between the characterization of the nitride surface of stainless steel and the hydrogen permeation behavior was evaluated. Some surface gas-nitride methods (nitride thickness was 1-3 μm) were effective to reduce the rate of hydrogen permeation of stainless steel by a factor of more than 10.

Plasma-sprayed coatings are widely used for thermal protection and wear stability of components and their importance with respect to structural durability is significant. These coatings feature an anisotropic porous structure as a direct result of the thermal spraying process. Although current literature provides methods for the identification of elastic properties of these materials, to our knowledge little research is dedicated to describing their plastic behavior, let alone to identifying the coatings’ plastic properties when subjected to macro-scale contacts encountered in industrial applications.

In this work we present a novel inverse method for the identification of elasto-plastic properties of thick plasma-sprayed coatings by means of macro-indentation, providing a macroscopic contact, and the Levenberg-Marquardt algorithm. In contrast to conventional instrumented indentation techniques that rely on a precisely measured load-displacement curve of indentation, the proposed method only requires knowledge of the residual indentation profile.

In this study we investigate various coatings featuring a thickness of around 150μm and degrees of porosity of 1.0-5.5%. For macro-indentation an experimental device based on a servo-hydraulic testing machine with a maximum capacity of 25kN was employed. Here, both spherical and flat-ended conical indenters were used. Interferometric profilometry was employed for 3D profiling of the residual indents, followed by deduction of a representative 2D-trace by azimuthal averaging of that profile around its central axis. For the determination of the coatings’ E-modulus and degree of porosity a conventional CSM micro-indentation instrument with different indenters and optical microscopy were used, respectively.

For a description of the coatings’ plastic behavior we made use of the Gurson-Tvergaard plasticity criterion coupled to a linear isotropic work hardening of the matrix. This criterion is appropriate for ductile porous solids as it takes into account the hydrostatic pressure, and it is readily implementable in Abaqus. The constitutive parameters to be identified include the yield strength σ_y0 and the work hardening coefficient K of the solid matrix as well as two dimensionless fitting parameters q₁ and q₂. These unknowns were successfully identified using the gradient-based Levenberg-Marquardt method that recursively updates input variables of the Abaqus-implemented FEM model to obtain a numerical indentation profile that sufficiently approximates the experimentally obtained one. We could show that the proposed method is reliable to identify the plastic properties of all investigated coatings.

Measurement of elastic constant € with good precision and accuracy is crucial for thin film applications. Accurate Young’s modulus of thin films can be used in calculating storage energy (Gs) and various industrial uses. TiZrN thin film was chosen to be the model material, because it remained single phase structure in the entire compositional range when deposited at temperatures below 500^oC and possessed superior mechanical properties compared with binary TiN and ZrN.

In this study, we proposed a standard procedure to determine the elastic constants of polycrystalline TiZrN thin films, by which the effects of composition on elastic constants were studied. The residual strain of the thin films was measured by cos²αsin²ψ X-ray diffraction method at multiple rotational angles (φ). From the slope of strain vs. cos²αsin²ψ plot, we can obtain average X-ray strain (AXS), which can effectively lower the statistical fluctuations. The residual stress of the thin films was determined using laser curvature method. Combing AXS from cos²αsin²ψ method, and stress from laser curvature method, we could obtain average effective X-ray elastic constant (AEXEC = E/1+υ).

In this study, TiZrN films with three different compositions were deposited on Si (100) substrate using unbalanced magnetron sputtering . The AEXECs were measured by the proposed method, and the effect of different Ti/Zr compositions was investigated. Since Young’s modulus is derived from atomic interaction in a solid, the variation in composition will lead to the change of interatomic force in TiZrN films, and thereby affecting the elastic constants. The variation of the resultant AEXEC with Ti/Zr ratio was discussed and correlated with the elastic constants of TiN and ZrN.

AlCrN based coatings becomes popular as a protective coating for various cutting tools in addition to TiAlN based coatings. It has been reported that AlCrN based coating shows better oxidation resistance and can retain mechanical property at elevated temperatures.

In this study, a series of AlCrN based coating was investigated their cutting performance and wear behavior. A series of AlCrN based coatings were deposited at 500C^Ο in nitrogen atmosphere by arc ion plating (AIP-SS002) under different compositions and deposition conditions. After the deposition, samples were subjected to standard compositional and structural analysis such as EDX, XRD and nano-indentation measurement for mechanical property. Cutting performance was determined by drilling tests, using WC-Co drills (MDS085SG, Sumitomo Electric Hardmetal). Regarding the cutting conditions, cutting speed is 75m/min., feed is 0.24mm/rev., depth of cut is 23mm, and work material is chrome molybdenum steel (AISI4140).