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April 1-3, 2008

METALFORM
Birmingham, Alabama

Booth No. 0931

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Salt Bath TD Process in Comparison With CVD, PCVD, and PVD

There are four kinds of thin hard coating processes that are being used worldwide to improve life of cutting tools, dies and molds, and machine components. There are CVD, PCVD, PVD, and TD process. Both the CVD and TD process started their wide application in Japan in advance of any other industrial countries including Germany where the CVD process was born. The PVD process came later. The largest current application overall is presumably being made by PVD because of it’s low temperature process. CVD and TD processes have larger application than PVD for the dies and molds and wear-resistant parts used in very severe working conditions such as forging, metal stamping with heavy gauge materials and materials that are difficult to formed.

The greatest life improvement of these kinds of dies and molds can be achieved not by PVD, PCVD, and CVD, but by TD. This is the opinion of a large numbers of users of these types of dies and molds. The TD process coating business in the USA is growing every year despite high treating fees and long turn-around times compared to other processes since it’s introduction. The TD process has been being used widely in Japanese industries since 1971. Many Japanese transplant companies in the US are using TD processed tooling imported from Japan or made in the US.

Irrespective of excellent performance of TD processed materials, the TD process seems to be at the bottom of the list when it comes to recognition in the world. There are some reasons for the lack of recognition. TD process could be done only under the license agreement with Toyota Central Research and Development Labs, Inc. (TRCDL), since TCRDL kept a number of patents in major industrial countries, there was so many companies to sell equipment to. However, the process requires simple and less expensive equipment than other processes. The salt used in TD process has very low vapor pressure at treating temperature and is not very toxic. Nevertheless, people misunderstand, that the salt bath method is dirty, danger, and poisonous, based on the knowledge on salt bath nitriding process and conventional salt bath hardening process for tool steels. There are a number of researchers for PVD and even CVD in the world, whereas just a few for the TD process. Simple equipment and simple operation seem to be not as attractive as a research theme to researchers in universities. TD is not competitive for PVD and CVD in application to metal cutting tools where both PVD and CVD have the largest application. It is because that TD process cannot make the coatings of nitrides and oxides that are indispensable for metal cutting tools to minimize the cratering of tool edges caused by reaction between tool materials and work materials under very high thermal loading by friction. The TD process work well for the metal cutting tools used under the cutting condition in which the cratering wear is not predominant and also for cutting tools for non-metallic materials.

The salt bath TD process needs a salt bath under air atmosphere. The salt bath is much cheaper in comparison with equipment for CVD and PVD equipment and furthermore, expensive cleaning equipment is not necessary. The coatings can be obtained only by immersing the substrate materials into the bath at hardening temperature for steels. Therefore, the process seems to be well suited for small industrial countries. Actually TD process has been introduced into Australia, Indonesia, Taiwan, India, Korea, South Africa, and Spain. University researchers have established new companies for TD process coating services in some of these countries. However, good knowledge for the substrate materials and heat treatment of steels are required to achieve a successful application in the coating service business for wide range of application fields. Wider freedom, comparing to PVD and CVD, in selection of substrate materials and the coating condition in relation to hardening of substrates and the possible distortion make people difficult to select the right condition in TD processing foe wide range of application. Application to only the limited fields is relatively easier. In fact, there are some companies in the world applying the salt bath process successfully onto only their products such as fastener making tooling, standard punches, textile machinery components, etc.

Salt bath TD process can easily make thicker coatings than PVD and CVD can. Not only the mono-layer of carbides, but also the multi-layered carbides and alloying carbides layers can be made by the TD process. However, these types of complicated coatings are not needed for most application of TD process. It is because that TD process is keeping the superiority in performance, with just single-layered vanadium carbide coatings, against multi-layered or alloyed carbides and nitrides coatings by PVD and CVD.

Simple water tanks filled with hot water are good enough to remove the salt and usually hot water tanks with stirrers are being used. However, need for washing to remove attached salt is one draw back of the salt bath process. This was one reason for developing the fluidized bed coating method based on the same principle of coatings formation. Vanadium carbide coating by using a fluidized bed replaced the salt bath vanadium carbide coating more than ten years ago in a standard punch maker in Japan. Previously one extra day of process time was needed for washing off the salt filled in small and long holes for ejection pins in punches. The fluidized bed process features easy automatic operation from loading, preheating, coating, quenching, and tempering to unloading with less expensive equipment investment comparing batch-type CVD and PVD equipment. Nitride coating is also possible on steel substrates with preliminary nitriding. This can be done at a temperature as low as temperatures for high temperature tempering of steels. This process has already been put into practical application in Japan. The salt bath TD coating at high temperature has large application in machine components including automobile for more than 25 years; only 10% of parts processed were tooling applications, this fact not being well known because of confidential character of application.

It is my opinion that both the fluidizing bed process and low temperature coating process have larger potential to such a large number of small machine parts when compared to CVD and PVD, and even the high temperature salt bath TD process. CVD will gradually lose the superiority in wear resistance application in the near future due to continuous growth of TRD process.