Estudo numérico da influência do coeficiente de atrito no ensaio de indentação em filmes finos

Abstract

A demanda por componentes mais resistentes ao desgaste e à corrosão tem promovido um interesse crescente da engenharia de superfícies que desenvolve processos para melhoria das propriedades tribológicas de materiais. O uso de revestimentos cerâmicos tem sido uma opção utilizada para melhorar estas propriedades mecânicas superficiais. Contudo se faz necessário o desenvolvimento de ensaios para avaliação do comportamento mecânico do filme e da sua interface com o substrato, o que requer equipamentos sofisticados, mão de obra especializada, acarretando altos custos operacionais. Com o avanço dos sistemas computacionais, a utilização da análise numérica para solucionar inúmeros problemas tecnológicos tem sido cada vez mais frequente e atualmente permite a sua implementação a baixos custos de processo. Neste trabalho, propõe-se a utilização do Método dos Elementos Finitos, através de um software comercial, para simular o ensaio de indentação com penetradores esféricos em conjugados compostos de substrato metálico e filme cerâmico com diferentes espessuras. Porém a utilização de técnica numérica para avaliar o ensaio de indentação em recobrimentos superficiais finos também tem apresentado desafios, principalmente na obtenção do comportamento mecânico do conjugado. Para melhor representar o ensaio de indentação foram considerados o efeito do coeficiente de atrito entre o penetrador e o recobrimento para, em seguida, serem analisados os campos de tensões e de deformações no conjugado para diferentes profundidades de penetração.Palavras chaves: Ensaio de Dureza, Filmes Finos, Elementos Finitos, Coeficiente de Atrito. ABSTRACTThe demand for components more resistant to wear and corrosion has promoted a growing interest in the surface engineering that develops processes to improve the tribological properties of materials. The use of ceramic coating has been an option used to improve the surface of mechanical properties. However, it is necessary to use tests to evaluate the mechanical behavior of the film and its interface with the substrate, which requires sophisticated equipment, skilled labor and consequently expensive cost. As computer systems have been improving recently, the use of numerical analysis to solve many technological problems has been increasingly frequent and current. Consequently, it allows its implementation with a low cost. This paper proposes to use the Finite Element Method (FEM) to simulate the indentation test with spherical indenters in conjugated compounds of metal substrate and ceramic film with different thicknesses, using commercial software. However, the use of this numerical technique to evaluate the indentation testing also has had problems in obtaining the mechanical properties of the conjugate. In this work, the numerical model also includes the effects of friction between the penetrator and the film to analyze the fields of stresses and strains along the different penetration depths.

Numerical Evaluation of Strength in the Interface during Indentation Spherical Testing in Thin Films

“…For this simulated case, Figure 4 depicts the numerical distribution of Maximum Principal Stress after unloading. This distribution of stresses in contact and interface regions shows the same behaviour as in earlier studies [1] [18]. Possible delamination, evidenced by the stress field formed in the interface region, was also observed.…”

“…In this analysis, different friction coefficient values were considered between the indenter and the sample, in a system with 3.00 µm-thick film and penetration depth of 20% of this thickness. These results show that the variation in the friction coefficient between the penetrator and the film surface has an influence on the distribution of the stress field in the film [1] [16] [18]. The same stress field behaviour was observed at the interface as a function of the decrease in modulus of elasticity, that is, the Maximum Principal Stress at the interface declines to low modulus of elasticity values at the interface.…”

“…The numerical simulation performed in this study used the finite element method and MARC™ software to model the indentation test with a spherical penetrator in a thin ceramic film deposited on a metallic substrate [11]. Due to the symmetry of the problem, asymmetric elements were used in the numerical model, significantly decreasing computational efforts [1]. The system studied was composed of an AISI 4140 low-alloy steel substrate coated with a chromium aluminum nitride (CrAlN) film, with the following thicknesses: 1.67 μm, 3.00 μm, 6.00 μm and 9.00 μm.…”

“…In order to obtain better distribution of field stresses and strains in the contact region of the indenter and at the interface with the substrate, a more refined grid was used in these regions ( Figure 2). The decrease in element size increases their number in these regions, raising computational costs, but reducing instability in the numerical result of the loading curve as a function of displacement in these simulations [1].…”

“…However, the mechanical properties of these films and their interface as substrate must be assessed. Instrumented indentation testing has been used to characterize these tribological systems [1].…”