Titanium Nitride Coating a bad Idea? 2

[sparkengy]

The company I'm doing an internship with has had problems with drill bits cracking. They are made from 17-4 PH Condition H900 and are coated in TiN.

Apparently the process of coating the bits is affecting the hardness and shape of the bits.

I've contacted several vendors to see if the coating process is a plausible explanation for the hardness issues, and have gotten mixed responses.

Is it possible the process of the TiN coating is affecting the hardness of the bit in a negative way? If so why? Would using 455 SS H900 be a solution?

Thanks.

 

[IRstuff]

You're EE, so you get some slack, but think about it. How does one get anything on a surface, and make it stick, and make it hard, without some sort of chemical, physical, or thermal processing?

Hardening of metals is usually accomplished by heat treatments and anneals. Mess with that, and you lose.

TiN is presumably a hard material. In order to get a hard material onto an oddly shaped surface, there would have had to been thermal changes.

Presumably, some people understand the issues and can tailor their processing to not degrade the performance of the substrate materials.

TTFN

FAQ731-376

 

[TVP]

sparkengy,

There are two processes for depositing TiN coatings: PVD and CVD. PVD process temperatures are in the range of 300-900 F (150-500 C) while CVD processes are much higher, in the range of 900-1100 C (1650-2000 F). PVD processes usually do not result in a hardness decrease or in dimensional change, while CVD processes certainly will reduce the strength of 17-4 PH in the H900 condition.

I suggest you determine which process was used, while also confirming the nature of the failure: brittle fracture? overheating? Coating delamination? There are numerous potential causes, and a full metallurgical evaluation should be performed in order to understand the failure mode. This usually costs less than $2000 for evaluating a handful of parts for hardness, microstructure, and fractography using SEM. Without understanding the root cause it is unlikely the just changing to 455 H900 will solve the issue.

 

[israelkk]

This is the first time I hear that drill bits are made of a PH stainless steel. Usually it is made of high speed steel or some kind of carbide. What they are intend to drill?

 

[gariartola]

Did drilling conditions change because the drill bit was coated? Coated tools are normally used in harder working conditions than the uncoated tools and sometimes the substrate can not stand them...

 

[tripleZ]

When I looked up 17-4 PH drills, I saw links to catheter drilling. What material are you cutting? Next, I'll second TVP on stating that you need to determine if the process is PVD or CVD. If the drills are PVD, then I'd probably start looking down the path of the machining conditions, the status of the drill after "X" cycles (look after 50, 100, 150 cycles and track edge condition), the conditions you're running under (coolant, no coolant, speeds, feeds, etc.), and the material for starters. Quite a nice DOE...=) Most of my issues with PVD drills to date have been coating peeling off (improper adherence to the substrate) and allowing the base material to get too hot, gall, and crack/break during a machining cycle. If you monitor the hole condition you'll be able to track this.

If it's CVD, you may look at sending these out to a lab for microstructural analysis. At the CVD processing temp, maybe you're seeing a pickup of nitrogen along the grain boundaries that's causing embrittlement in the grain structure? That's where a metallurgist will help.

 

[sparkengy]

Israelkk,
These bits are used in the medical field during knee replacements.

Everyone,
Thank you all for the information and suggestions

 

[mighoser]

Figure out why your drill failed. Overstress condition or material embritlement. I would say lower your hardness on the 17-4 and do a liquid salt bath nitride. Use H1075. Or do the same and keep the TiN coating. We supply these coatings using PVD setup and can operate as low as 100C. We also do PACVD coatings like diamond like carbon (DLC) which is even lower in processing temp. We've coated polycarbonate and other polymers in DLC. If you want to switch to another PH stainless, I'd try Custom 465 in the H1000 condition or even cold worked.