ICMCTF2011 Session E2-3: Mechanical Properties and Adhesion

The proper design of wear resistant coatings applied to cutting tools comprises the optimization of the mechanical properties (Young's modulus, yield strength, adhesion, intrinsic stresses, fracture, fretting and wear resistance etc.) of the coating-tool system.

The goal is to find material and structural solutions which keep the resulting stress strain field under typical application conditions below the stability limits of the system.

Based on nanoindentation measurements obtained from the coating-tool system which should be optimized, a scratch test is dimensioned with respect to load range and indenter geometry. The measured data from this “Physical Scratch Test” are used to simulate spatial stress profiles and to calculate the von Mises stress characteristics and the maximum tensile stresses in the scratch direction. In a further step, the simulations are used to suggest scratch parameters (“Fine Tuned Scratch Test”) which increase the sensitivity of the test for specific depth regions in the coating-tool architecture and allow improved and more sensitive investigations of critical interfaces, transition layers and surface-near substrate regions.

The tests were performed at PVD coated inserts (nitrides and oxides) and compared with the results obtained from cutting tests.

The scratch resistance of coatings and the adhesion between coating and substrate is usually determined in model experiments preceded with sharp diamond indenters. These common methods as described for example in EN 1071-3 often fail for the combination of hard coatings on soft substrates due to very small critical loads. Hence in the industry are used a lot of highly subjective and provisional test methods. On the one hand quantitative comparability is difficult with common methods since defects already occur at very small loads for ductile and relative soft substrate materials like plastics. On the other hand wear of the indenter in contact with hard coatings like pure diamond-coatings requires its cost-intensive replacement.

In this article a macroscopic tribological-mechanical test method is suggested which uses balls of hardened steel as indenters. A wide scope can be applied for both soft and hard coatings and different substrate materials. By variation of the ball-diameter, the normal contact force and the sliding speed different levels of stress and wear can be created to analyse the tribological and mechanical behaviour between body and counterpart as well as the interface of coating and substrate. The method is also suitable for timesaving pilot tests to find out the proper ball size and load for following ball-on-disk-tests on a tribometer. To determine scratch resistance close to reality as usual scratch conditions on consumer products are better represented by a small ball than a sharp diamond indenter. Another benefit of the presented test method is the cost saving acquisition of the balls for indentation in very high quality as they are standard parts in the ball bearing industry. For every test on very hard coatings a new ball can be used with the possibility to detect the wear both on the base object and the counterpart. The occurring failure modes of coating and substrate can be compared also with comparatively easy numerical models to verify the results. Additional to the test concept first results of different coatings will be presented in this paper and compared with the results of common scratch tests.

In films thinner than about 200 nm, yield stress plateaus or even decreases, clearly calling for an alternate relaxation mechanism. Parallel glide of dislocations, a potential symptom of enhanced grain boundary diffusion, is one of the possible substitution process, but so far, it has never been observed elsewhere than in Cu films [3].

Here, we will show that in situ TEM can display the dislocation mechanisms while they operate in thin films and thus directly corroborate or contradict existing theories. Examples will be taken from Al and Cu films on rigid substrates. The stress is induced by the difference of CTE between the film and the substrate, as in wafer curvature experiments. We found that, for thicker films, amorphous interfaces play the role of dislocations sinks, similarly to free surfaces [4]. The increased strength of metallic films with amorphous interfaces is thus dictated by the nucleation of fresh dislocations. In thinner films, scarce dislocation activity is clearly replaced by other processes. Diffusion processes are harder to observe and quantify [5], but plasticity can also be carried out by grain boundary migration[6], similarly to what has been recently observed in nanocrystalline metals [7, 8], and despite the presence of the substrate.

[1] Nix WD. Metall. Trans. A 1989;20A:2217.

[2] Dehm G, Balk TJ, Edongue H, Arzt E. Microelectronic Engineering 2003;70:412.

[3] Balk TJ, Dehm G, Arzt E. Acta Mat 2003;51:4471.

[4] Legros M, Hemker KJ, Gouldstone A, Suresh S, Keller-Flaig RM, Arzt E. Acta Mat 2002;50:3435.

[5] Legros M, Dehm G, Balk TJ, Arzt E. Science 2008;319:1646.

[6] Mompiou F, Caillard D, Legros M. Acta Mat 2009;57:2198.

[7] Rupert TJ, Gianola DS, Gan Y, Hemker KJ. Science 2009;326:1686.

[8] Gianola DS et al. Acta Materialia 2006;54:2253.

In this work, finite element (FEM) and experimental studies were conducted to understand adhesive and cohesive failures in coated systems with thin film residual stress gradients. The FEM analyses considered a rigid indenter applying normal and tangential loads on the coated systems. Three different cases of variation of film compressive stresses were considered. Stresses: (i) increased from the surface to the film/substrate interface; (ii) decreased from the surface to the film/substrate interface or (iii) were uniform along film thickness. In the numerical studies, the mechanical behavior of both coating and substrate was considered elastic-perfectly plastic. In the experimental portion of this work, a set of depositions of titanium nitride thin films was conducted varying the value of substrate bias during deposition, as an attempt to experimentally prepare specimens with the same trend of residual stress gradient as those analyzed numerically. These specimens were later submitted to scratch tests in order to analyze the critical loads obtained in each case. Numerical results provided a comparison of the distribution of stresses obtained in each simulation, indicating a trend of easier film debonding for the conditions where the stress values were more compressive at the interface and decreased towards the surface. This result is in qualitative agreement with the experimental study, in which lower critical loads were observed for the films where deposition started with high values of substrate bias and the bias was reduced as the deposition proceeded.

Nanocrystalline electroless nickel-boron deposits were synthesized on steel substrates and submitted to heat treatment under non-reactive atmosphere to enhance their properties. Their mechanical and tribological properties were investigated by various methods including nanoindentation, Taber wear testing and scratch tests. Their structural properties were also studied.

The hardness of the deposits increased from 900 to 1250 hv₁₀₀ due to optimal crystallization of the nanocrystalline coating while the Taber wear index was halved after heat treatement.

The scratch tests resistance of the coatings was good in both as-deposited and heat treated conditions.

Interface adhesion is one of the foremost important properties of diamond-coated tools, critically linked to their machining performance. Despite of significant advances in diamond deposition technologies that can produce superior diamond properties, coating-substrate adhesion remains the major challenge to extend tool life and understanding the adhesion characteristics are of primary interests to coated-tool makers and users.

In this study, a micro-scratch tester was applied to evaluate the adhesion of diamond coated carbide tools, about 4 µm thickness, at the room temperature. A diamond indenter with a tip radius of 50 µm was employed. The scratch speed was 2 mm/min with the progressive loading method. The scratch length for each test was 5 mm. The maximum normal forces applied were 30 N. During the testing, tangential force, acoustic emission (AE) signals, and depth of the scratch were acquired. Scratch marks and delamination areas were examined by digital microscopy, white-light interferometery and scanning electron microscopy.

The results indicate that the onset of delamination can be clearly captured from the force and AE signals. Though scratch testing may provide information such as critical loads, the interface characteristics cannot be directly utilized when evaluating interface behaviors of a diamond-coated tool subject to tribological loading. Therefore, a finite element analysis (FEA) model was developed to simulate the scratch process with the interface modeled by a cohesive zone. The results indicate that it is feasible to use FEA combined with scratch tests to evaluate the interface properties, i.e., the cohesive zone strength and characteristic length.

[1] Shan, Z. W. et al. Ultra high stress and strain in hierarchically structured hollow nanoparticles. Nature Materials 7, 947-952 (2008).

In the described investigations, platinum (Pt)- iridium (Ir) coatings were deposited directly on cemented carbide samples by Physical Vapour Deposition (PVD) process. Moreover, for improving the adhesion, different materials such as of Ni and Cr were employed as adhesive interlayers at various thicknesses. These interlayers were deposited on the substrate before the Pt-Ir film, during the same PVD process. Appropriate experimental procedures were conducted for characterizing the coatings’ mechanical and adhesion properties such as nanoindentations, nano-impact and nano-scratch tests. FEM calculations simulating the films’ loadings during nano-impact test explain the effect of the adhesive interlayer on the entire coating substrate structure strength.