Citation: Wachowicz, J.; Wilkowski, J. Influence of Diamond Grain Size on the Basic Properties of WC-Co/Diamond Composites Used in Tools for Wood-Based Materials Machining. Materials 2022, 15, 3569. https://doi.org/10.3390/ma15103569 Academic Editors: Robert ˇ Cep and Irina Hussainova Received: 28 February 2022 Accepted: 12 May 2022 Published: 17 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Influence of Diamond Grain Size on the Basic Properties of WC-Co/Diamond Composites Used in Tools for Wood-Based Materials Machining Joanna Wachowicz * and Jacek Wilkowski Department of Mechanical Processing of Wood, Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences, Nowoursynowska Street, 166, 02-787 Warsaw, Poland; jacek_wilkowski@sggw.edu.pl * Correspondence: joanna_wachowicz@sggw.edu.pl Abstract: The paper presents the effect of diamond particle size (varying between 2.5 μm and 20 μm) on the microstructure, density and hardness of WC-Co/diamond composites. The obtained materials contained 30% vol. diamond. The advanced sintering method Pulse Plasma Sintering (PPS) was used for the production of composites. The sintering process was carried out in two stages at a pressure of 50 and 100 MPa and a temperature of 1050 ◦ C. Depending on the size of the diamond particles, composites with a density of 91–99% were obtained. Microstructure studies were performed employing scanning electron microscopy, along with an analysis of the chemical composition in micro-areas. Additionally, the phase composition was investigated by means of X-ray diffraction. In addition, hardness tests were performed. It was found that the size of the diamond particles significantly influenced the microstructure of the tested materials, as well as the density and hardness. As a result of PPS sintering of composites containing the finest diamond particles (2.5–5 μm), the presence of a metastable type of diamond—graphite was found. Keywords: sintering; cemented carbides WC-Co; powder metallurgy 1. Introduction Sintered carbides WC-Co are widely used tool materials. These materials are charac- terized by high hardness and resistance to abrasive wear. Due to the development of the furniture industry, the demand for new tool materials has increased in recent years, provid- ing increasingly higher machining parameters and improved performance. The researchers’ works are focused mainly on improving the cutting and strength properties of WC-Co car- bides, by: hard coating, ion implantation or development of new composite materials based on WC-Co [1–15]. Despite all efforts of materials engineering, the “ideal tool material” has not yet been developed. A WC-Co composite with dispersed diamond particles could be the material that would ensure appropriate processing parameters and high durability at a relatively low production cost. Carbide matrix would ensure high resistance to abrasive wear and brittle fracture whereas the diamond particles—high hardness. Producing a WC-Co composite with diamond is not simple. Diamond is a metastable, allotropic variant of carbon and undergoes a transformation into a stable phase that is graphite easily. It is only at a pressure within 3–12 GPa that the stability area for the diamond occurs. The process of graphitization is favored by metals from the VIII group of the periodic table (Co, Ni, Fe). Moreover, this phenomenon increases with the growth of temperature and time of sintering. The activation energy of the transformation of diamond into graphite also depends on the internal and external crystal structure. The stability of diamond is negatively affected by defects in the diamond crystals in the form of metallic and non-metallic inclusions, originating mainly from the catalyst materials from the diamond synthesis process. The rate of transformation of diamond to graphite also depends on the annealing atmosphere [16]. Materials 2022, 15, 3569. https://doi.org/10.3390/ma15103569 https://www.mdpi.com/journal/materials