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Will titanium nitride coating make hardened steel more brittle?

December 18, 2009

We have machines that use a forming die to bend steel rule. They've been in production for years. The forming die is hardened to Rockwell 60, with no coating. The only issues we've had with these dies through the years is wear. Periodically, the machines need to be re-calibrated to account for wear. After a year or so, a new forming die needs to be purchased. Just recently, the manufacturer has started coating these dies with titanium nitride. Since March of '09 (the first coated die sent to us), we have broke every coated die (eight total dies). The manufacturer says nothing has changed with the die except the coating, One die lasted as long as two months and another die eight hours, before breaking. Looking from the top, the die is shaped like a piece of pie (cherry). The steel rule is placed in one side of the die and bent around the point at varying degrees. The area of breakage is in the nose of the die. In the last 10 years we have never broke an uncoated forming die. The manufacturer says the material, hardness, machining process is all the same and is the same as far as I can tell...So, will titanium nitride coating make hardened steel more brittle?...I have other failure theories but none directly involving the coating.

Lee Argo
solving problems at work - Kansas City, Missouri

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Because TiN is deposited at high temperatures -- around 450 °C -- in general the metal is annealed some, so it should be less brittle. However, the coating itself is very brittle. Under high tensile loads it will crack. It could be that the coating is cracking, and cracks in the coating are propagating down into the metal.

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego,
California
January 1, 2010

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Your thought gives a little more credence to another theory I had. The theory is; as an uncoated die wears, it slowly changes the stress points around the nose of the die. A coated die doesn't wear as quickly and maintains the same stress point...combine that theory with your thoughts... Because of a crack developed in the coating, from tensile loading, the crack becomes the permanent stress point. The crack from the coating propagates through the die and ultimately causes die failure.

Another coating question-Will a TiN coating be consistent throughout a 1 mm x 25 mm x 30 mm deep slot?

Lee Argo
- Kansas City, MO USA
January 5, 2010

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You'll have no coating on the sides of the slot beyond the first few mm.

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego,
California
January 5, 2010