Small-scale testing of micromechanical response of cemented carbides

Resumen

Cemented carbides are composite materials widely used in different industry fields within applications involving wear, due to their outstanding wear resistance. The most commonly used are WC-Co grades, for Co wettability with the carbide and adhesion characteristics. Emergence of new applications, the existence of advanced characterization techniques, economic and environmental aspects, among others, encourages the development of a new cemented carbides generation containing other binding phases as Ni and Fe or alloys of them. Furthermore, Co powder has been classified as very toxic for the human health and the combination carbide-cobalt hardmetals dust has shown to be even more toxic than both pure cobalt and tungsten. The success of substitution of the main constituents of cemented carbides, have been commonly measured in terms of their final mechanical properties at macroscale such as hardness, toughness and transverse rupture strength; and structural integrity under service-like conditions, such as corrosion resistance, thermal shock and fatigue resistance. In this sense, general framework of microstructural effects – carbide mean grain size, volume fraction and chemical nature of constitutive phases - on the mechanical response of cemented carbides is well established at the macroscale. However, assessment of the individual role of the binder and carbide phases at local scale i.e. microscale, is yet to be studied in depth. Within micromechanical testing, special attention has being paid to the micropillar compression approach because its advantages: the stress-state is nominally uniaxial, allowing a straight conversion of the measured load-displacement data into flow curves; sample preparation by means of Focused Ion Beam (FIB) milling is a relatively easy machining route; it involves the use of a conventional nanoindenter with a flat-end tip; and, it can be performed ex-situ or in-situ by using Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) techniques. However, attention have to be paid to sample sizes since it has been well established that intrinsic properties of crystalline materials such as yield stress and strength, can be greatly influenced by extrinsic factors such as volume. For instance, results have evidenced an inverse relation between hardness and the indentation depth at the micro- and nanometric length scales. Regarding cemented carbides, recent studies showed that changes in volume fraction of binder and carbides in samples can lead to wide scatter in results of Young’s modulus measured at the microscale. Following the above ideas, in this PhD thesis uniaxial compression of micropillars and nanoindentation have been selected to evaluate the role of binder and carbides regarding their chemical nature and microstructural dimensions, i.e. carbide mean grain size and binder mean free path, in the mechanical properties and response of cemented carbides at local scales. This thesis is presented by a compendium of scientific publications in which several specific objectives are studied individually. In the first and second publications the sample size and the volume fraction of constitutive phases within the micropillar are studied respectively. Results allowed to overcome the size effect issue – usually found when testing in the micro or nanometer regime – by selecting an appropriate sample size, to accomplish reliability on the mechanical properties evaluated at local length scales. Third and fourth publications are devoted to investigating the mechanical properties of cemented carbides with partial or total substitution of WC or Co as main constitutive phases based on their intrinsic mechanical properties and behavior. Outcomes evidence that small scale testing of complex composite materials such as cemented carbides by means of uniaxial compression of micropillars and nanoindentation, allows to evaluate the role of each constitutive phase on their mechanical behavior.