Giuseppe Pintaude

An approach to elucidate the different response of PVD coatings in different tribological tests

Abstract In this paper, the different performances of duplex (i.e. with a prior plasma nitriding treatment) and non-duplex PVD coatings in micro-abrasive wear tests and impact tests are discussed, taking into account some key properties, such as nanohardness ( H ), elastic modulus ( E ) and the H / E ratio. The nanohardness and elastic modulus of the coatings were determined by nanoindentation and were characterised by means of several analytical methods, such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, Knoop hardness measurements, scratch tests and scanning electron microscopy (SEM). Similar H / E ratios were found for all coatings under investigation. Duplex (Ti,Al)N coatings exhibited the best wear resistance in micro-abrasive wear tests. The results obtained in such tests suggested the occurrence of abrasive wear by plastic deformation, with the hardest coatings displaying the lowest wear rates. The highest wear rates were recorded for duplex and non-duplex Cr–N coatings. Conversely, a duplex Cr–N coating exhibited the best performance in impact tests using a tungsten carbide ball. The duplex Cr–N coating displayed the lowest elastic modulus among all coatings tested, which can be attributed to the presence of a small amount of α-Cr metallic phase. This coated system also showed the smallest difference between the substrate and coating elastic moduli, indicating that low elasticity mismatch between the coating and substrate is desirable for achieving good performance in impact tests. The differing behaviours exhibited by duplex (Ti,Al)N and Cr–N coatings in different tribological tests reveal the importance of selecting the best coating for a given tribological application.

The particle size effect on abrasive wear of high-chromium white cast iron mill balls

Granite grinding tests, under dry and wet conditions, were performed to assess the influence of abrasive particle size to the wear behavior of martensitic high-chromium white cast iron mill balls. The tests were performed, at first, using raw granite particle sizes between 0.074 and 19.1 mm, and then with coarse and fine granite fractions obtained after screening the raw granite in a 3.36 mm sieve. It is demonstrated that the relative particle/ball size relationship is the determining parameter to ball wear. The highest ball wear rates were observed for fine granite grinding under dry (120 mg/cycle) and wet (129 mg/cycle) conditions. The lowest wear rate (ca. 50 mg/cycle) was observed for coarse granite grinding (dry and wet). These different results were attributed to the different size relationships between grinding body diameter and granite particles size. For wet-grinding of raw granite, the mineral components may influence significantly the wear behavior. Feldspar can act as a bonding agent, gluing fine quartz particles to the coarse granite and to the balls surface and turning the dependence of the relationship between the relative sizes of ball and granite particle less important to the wear process. This explains why wet-grinding of raw granite results in a ball wear two times greater (106 mg/cycle) than dry-grinding (51 mg/cycle).

Abrasion and corrosion resistance of new Ni-based coating deposited by HVOF thermal spray process

Abstract Coatings based on NiCrAlC intermetallic based alloy were applied on AISI 316L stainless steel substrates using a high velocity oxygen fuel torch. The influence of the spray parameters on friction and abrasive wear resistance were investigated using an instrumented rubber wheel abrasion test, able to measure the friction forces. The corrosion behaviour of the coatings were studied with electrochemical techniques and compared with the corrosion resistance of the substrate material. Specimens prepared using lower O2/C3H8 ratios showed smaller porosity values. The abrasion wear rate of the NiCrAlC coatings was much smaller than that described in the literature for bulk as cast materials with similar composition and one order of magnitude higher than bulk cast and heat treated (aged) NiCrAlC alloy. All coatings showed higher corrosion resistance than the AISI 316L substrate in HCl (5%) aqueous solution at 40°C.

Introduction of the Ratio of the Hardness to the Reduced Elastic Modulus for Abrasion

Modeling the wear rate is a complex process. The several possibilities of chemical, physical and mechanical changes at the interface are the most probable reasons for this [1]. In this manner, it is reasonable to consider the wear rate as a stochastic process [2, 3], and indeed this approach was taken into account by Archard [4], when he formulated his well-known model. Since then, the majority of available models are based on his proposition, independent on the characteristics of mechanical system. Considering a sharp contact, both Torrance [5] and YiLing & Zi-Shan [6] for sliding and rolling abrasion, respectively, modified Archard’s equation based on elastic effects, and the ratio of the hardness (H) to the Young’s modulus (E) was the main parameter of the models. In a tribological system with dissimilar materials, for example a ceramic abrading a metal, one material can experience the yielding and the other the brittle failure. This difference in the mechanical behaviors can be decisive for the final performance to wear.

Relation between strain hardening exponent of metals and residual profiles of deep spherical indentation

Abstract Strain hardening exponent is an important mechanical property usually obtained from tensile tests, which implies that a specific specimen preparation and long routines of calculus should be performed. An alternative way to obtain this property is the use of spherical indentation hardness, measuring the profiles of indentation morphology: piling up or sinking in. In the present investigation, the indentation morphologies observed after tests with a spherical indenter for aluminium alloys (AA 6063-T5 and AA 1350) and steels (AISI 1020 and AISI 316L) are presented. Indentation tests were performed with different sphere diameters and test loads, to obey the Meyer law and to keep constant the relation between load and indentation diameter, varying the plastic strain level. Tensile tests were performed to make use of reference values. The residual profiles were obtained using a two-dimensional profilometer. The results allow discussing the range of validity of several models proposed in the literature. For some test conditions, 316L stainless steel and 1350 aluminium alloy partially recovered present an unexpected behaviour, which the models are unable to predict.

An Application of Mean Square Calculus to Sliding Wear

In this paper the Archard model and classical results of mean square calculus are used to derive two Cauchy problems in terms of the expected value and covariance of the worn height stochastic process. The uncertainty is present in the wear and roughness coefficients. In order to model the uncertainty, random variables or stochastic processes are used. In the latter case, the expected value and covariance of the worn height stochastic process are obtained for three combinations of correlation models for the wear and roughness coefficients. Numerical examples for both models are solved. For the model based on a random variable, a larger dispersion in terms of worn height stochastic process was observed.

A review on the reverse analysis for the extraction of mechanical properties using instrumented Vickers indentation

This work presents a review on recent methodologies for the analysis of data obtained through instrumented indentation testing. Experimental tests, using a Vickers indenter, were carried out on low-carbon and bearing steels and indents were later analyzed in a laser interferometer. The results were used to verify the accuracy of methods proposed to predict the indentation morphology, pile-up or sink-in, and the accuracy of routines proposed to extract the mechanical properties of the indented materials. The occurrence of pile-up in all tested materials indicated that models may fail in predicting this behavior and, consequently, in determining the yield stress and strain-hardening exponent.

Experimental analysis of indentation morphologies after spherical indentation

Indentation morphologies depend on the mechanical properties of materials, especially the strain-hardening exponent and yield strength-to-elastic modulus ratio. Hernot 1et al. described a model that can be used to obtain the indentation morphology index from properties determined in tensile tests. The model is used here with two aluminum alloys and 1020 steel tested under spherical indentation with different loads and ball diameters. There was good agreement between the values predicted by the model and the experimental findings for all the materials tested except partially recovered AA1350 aluminum alloy (H24 condition). This exception is discussed and a possible explanation for it is sought in other experimental deviations and in microstructural inhomogeneities.

Characteristics of Abrasive Particles and Their Implications on Wear

Geometry, hardness and size can be considered the most important characteristics affecting the wear rate caused by abrasive particles. The combined effect of these characteristics on wear has not yet been modeled as a whole. One particular model, proposed by Rabinowicz et al. (1961), involves a single particle being considered as a pointed tool, so that the effect of the geometry can be determined.

An Overview of the Hardness Differential Required for Abrasion

This paper presents an overview of the hardness differential required for abrasion. Empirically, the abrasive must be at least 1.2 times harder than the worn surface if it is to produce a scratch. This value has been determined theoretically using slip-line field modeling, which assumes rigid-plastic mechanical behavior, an assumption that is inadequate for most abrasive particles. Two approaches using elastic-plastic models and three tribological pairs with similar ratios of abrasive hardness to worn material hardness were tested to gain an understanding of the hardness differential required for abrasion. The analysis showed that the ratios of the property of the abrasive to the property of the worn surface did not change with the model used when the mechanical behavior of the materials was similar. However, when the behavior of the materials was very dissimilar—as is often the case in abrasive processes—the ratios varied greatly depending on the model used, showing that there is a need for models to describe the hardness differential required for abrasion.