“Diamond, the hardest substance known to man.” Advertisements touting this quality of diamond are used to represent many different types of products, and in its truest sense, it implies a material that is unequaled in terms of strength and resilience. However, while it is true that diamond is the hardest substance known to man, special care is needed with diamond tools in order to optimize this characteristic.
The diamond cutting tools that are used in machining and sawing applications begin as individual diamond crystals sintered along with a carbide backing in a high pressure high temperature environment. At temperatures and pressures simulating those deep in the center of the earth, the cobalt material in the carbide liquefies and facilitates the growth or bonding of the individual diamond crystals to each other. From the joining or sintering of these crystals, we derive the term polycrystalline (many crystals) diamond, also referred to as PCD. After the material is cooled, it is finished in a disc form before being cut into the various shapes required for tips on cutting tools and saw blades.
The tips consist of a diamond layer that is approximately .5mm in thickness supported by the carbide so that total thickness is generally 1.6mm. A variety of other thicknesses can be specified from the manufacturers with specialized diamond layers as needed. This tip is then brazed to the tool body for finishing. Throughout the processing from disc to tip and onto the finishing of the tool, the utmost care is taken to preserve the diamond material.
The major causes of concern for the diamond material are chipping, leaching and overheating, all of which can lead to premature failure of individual tips, and ultimately the entire tool. Leaching can occur throughout the entire processing of the diamond material from initial sintering up to and including the use as a finished tool in the field. The leaching results when the cobalt material is exposed to acidic chemicals used in cleaning or processing of the tool as well as during the final machining operations. Oftentimes, the cause of the leaching is an innocent change in cleaning materials. Some detergents are more acidic than others which when exposed to the cobalt for extended periods result in leaching of the cobalt binder. Other less invasive causes are the cleaners used within machines that utilize water based coolants. Many times these water based coolants will have a pH less than 7 (neutral pH) and into the 4 and 5 ranges. To put this in perspective, normal household vinegar is about a 3 on the pH scale and lemon juice would be in the range of 2. Leaching can occur on some rate at any pH below 7. Ultimately the leached surface provides less support for the diamond materials which result in premature failure due to chipping. An example of a leached surface is shown in the picture on the left which resulted in an undercut along the top edge, and the picture on the right shows a more dramatic example where the entire surface has been leached.
Chipping is another defect that leads to premature failure. Most companies processing diamond, and utilizing diamond tools have experienced the durability of carbide tools somewhere in their operations. The habits formed with the carbide tools are carried over to these diamond tools with little thought given to the brittle nature of the diamond itself. Again, we are all accustomed to hearing it is the hardest substance known to man. The reality is that the diamond edge is especially prone to chipping. It is so brittle, that care is taken to not use micrometers when checking dimensions of the tool during processing to avoid chipping the cutting edge. Many facilities are use to operators tossing their carbide tools and tool assemblies onto a bench or into a tool box. Diamond or PCD tools will not tolerate this kind of treatment without chipping. Chipped surface quickly deteriorate causing problems with finish and tool life in the operation. An example of chipping is shown in the two pictures below.
Overheating is another major cause of tool failure for PCD tools. The majority of the time this overheating takes place during the brazing operation when re-tipping a tool that is being serviced. The PCD or diamond material begins back converting to its natural carbon state with temperatures of approximately 700°C. If exposed to high heats for longer period of times at slightly lower temperatures, we can also see this degradation occur. Cracking of the material can also result from overheating as stresses relieve in both the carbide and diamond layers at different rates. Examples of overheated tools are shown below.
By educating everyone that comes into contact with PCD tools, on the proper care and handling of these materials, it is easy to realize 20 to 100 times the life of carbide tools (depending on application). Clean with mineral spirits instead of highly acidic detergents to decrease the opportunity for cobalt leaching. Handle the tools carefully making sure to avoid contact with each other and with other items before and after operation. Utilize induction brazing instead of torch brazing or utilize pyrometers to avoid overheating. Diamond is the hardest substance known to man, but still requires proper care to maximize its benefits.
Source: Vollmer of America Corp. Visit Vollmer-US.com or call (412) 278-0655.
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