Mechanical strength of ground WC-Co cemented carbides after coating deposition

Abstract

Manufacturing of hardmetal tools often involves surface grinding, ion etching and final coating.Each stage throughout the manufacturing chain introduces surface integrity changes which may be critical for defining the final mechanical behavior of the coated tools. Within this context, an experimental test program has been developed to assess the influence of a coating (TiN) deposition on surface integrity and transverse rupture strength of a previously ground finegrained WC-Co grade substrate. Four different substrate surface finish conditions (prior to ion etching and coating) were evaluated: as sintered (AS), ground (G), polished (P), and ground plus high temperature annealing (GTT). Surface integrity and fracture resistance characterization, complemented with a detailed fractographic analysis, were performed on both uncoated and coated samples. Results show that the surface integrity after grinding has been partly modified during the ion etching and film deposition processes, particularly in terms of a reduced compressive residual stress state at the substrate surface level. Consequently, the grinding induced strength enhancement in hardmetals is reduced for coated specimens. Main reason behind it is the change of nature, location and stress state acting on critical flaw: from processing defects existing at the subsurface (uncoated G specimens) to grinding-induced microcracks located close to the interface between coating and substrate, but within the subsurface of the 1 Current address: AMES-Sintered Metal Components, Camí de Can Ubach, 8, Pol. Ind. ¨Les Fallulles¨, 08620 -Sant Vicenç dels Horts, Barcelona -Spain latter (coated G specimens). This is not the case for AS and P conditions, where flexural strength does not change as a result of ion etching and coating. Finally, fracture resistance increases slightly for GTT specimens after coating process, possibly caused by a beneficial effect of the deposited film on the residual stress state at the surface.

Grinding-induced metallurgical alterations in the binder phase of WC-Co cemented carbides

“…The residual stress depth profile for sample G has been reported previously [15,19]. The grindinginduced residual stresses extend approximately 12 μm below the surface, i.e.…”

“…In recent publications, authors have shown that grinding induced microcracks exist down to a depth of about 2 μm, and microcracking followed two different microstructural paths: either WC/Co interfaces or transgranular within the carbide phase, whereas the former being more predominant than the latter for the same material and grinding conditions used in the current study [15,18,19].…”

“…However, similar to other hard and brittle materials [8][9][10], as a result of the abrasive operation, surface integrity of hardmetals is altered at both surface and subsurface levels, particularly in terms of microcracks, residual stresses, and metallurgical alterations within the metallic binder. Thus, mechanical and tribological performance of corresponding tools and components are also affected [6,[11][12][13][14][15][16][17][18][19].…”

Experimental Investigations of Micro-Meso Damage Evolution for a Co/WC-Type Tool Material with Application of Digital Image Correlation and Machine Learning

“…For applications of hard metal components, such as cutting tools and drawing dies, the cemented WC-Co share about 98% of the market today [24]. For such materials, investigations on strengths, hardnesses and micromechanical properties can be found, e.g., in [25][26][27][28][29][30][31][32][33][34][35][36][37]. To further improve the hardness, diamonds can be added, and coatings, e.g., Ti or Ti-N coatings, can be used to enhance the bonding between diamonds and the base material (WC-Co).…”