An Empirical Research Report
Timothy J. Johnson
April 12th, 2007
ME595 – Manufacturing Tribology
Diamond is the hardest material known to man, in this day and age. Typically, diamonds have been known to demonstrate hardness values up to 12000 HV (Vickers Hardness) or approximately 100HRC. Diamond-coated Tungsten Carbide tools have demonstrated improved machining characteristics over coated tool steels commonly used today. With the increased use of composites, ceramics, and other ultra-hard / lightweight materials in numerous industries, diamond-coated machine tools are becoming more common since their performance improvements generally out weigh their increased cost.
This paper is an empirical summary of various research papers related to diamond coating processes of machine tools and their applicability to current and advanced machining processes. Topics to be discussed are: Diamond Coating Process Overview, Applications of Diamond Coated Tooling, Diamond Coated Tooling Machining Capabilities, and some conclusions on the topic. Though a few different tool coating processes are mentioned in this paper, the main focus is on CVD (Chemical Vapor Deposition) Diamond Tooling and its advances in the manufacturing industry. Diamond Coating Process Overview
Since natural diamonds are very expensive, the machining industry has developed a multiple methods of creating man-made or synthetic diamond materials. These synthetic materials are then bonded to a substrate tool before they are suitable for machining. Discussed in this paper are threes different bonding techniques: Bonded Diamond Grit, PCD (Poly-crystalline Diamond) Diamond Tools, and CVD (Chemical Vapor Deposition) Diamond tools.
Bonded Diamond Grit Tools
Bonded diamond grit tools are the most common diamond-coated machine tools used in the industry. Their basic construction consists of crushed synthetic or natural diamond particles glued to a variety of cutting, grinding and polishing tools. They are primarily used in the mining industry, but are also used for edge-finishing of carbon fiber composites and grinding hard materials like glasses and ceramics .
PCD (Poly-crystalline Diamond) Coated Tools
PCD tools are similar to bonded diamond grit tools in that they both start with fine diamond particles. However, the PCD method involves forming the particles into a dense sintered material using a binder made from cobalt. These cemented compacts are then brazed into various shapes (for different kinds of cutting tools) and brazed onto solid carbide tool bodies. The PCD tools edge can then be ground down to a finished shape and even reground for tool sharpening purposes. PCD tools are over 50 years old and are prevalent in the aerospace industry for drilling rivet holes into carbon fiber composite materials. The automotive industry also uses these tools for machining of aluminum composites which contain high silicone content (e.g. Al 390). These tools can be expensive, but are often the best option for very high precision cuts and high impact loading .
CVD (Chemical Vapor Deposition) Diamond Tools
The main focus of this paper will be on CVD tools. Using CVD technology, diamonds are actually grown (nucleated) onto the surface of a machine-tool substrate. For this process to be successful, careful attention must be paid to the substrate material selection. The process requires prolonged exposure to high temperatures so tungsten-carbide or ceramic tooling substrates are preferred. For optimum coating adhesion, C-2 grade tungsten-carbide must be used .
Also, the parts must be carefully prepared before attempting the CVD process. Typically, the parts are cleaned thoroughly, and then put through a 2-step chemical preparation process. As shown below, in Figure 1, the first step involves roughening up the surface of the tool substrate for improved...