PVD and CVD Coatings on Tool Steels
A coating raises the hardness of the tool surface, lowers the friction between tool and workpiece, and pushes back the point at which the tool has to be resharpened. Which coating process can be used on a given tool steel comes down to one number, the temperature at which that steel was tempered. Chemical vapor deposition runs above 800 °C (1472 °F), so the tool has to be tempered again after coating. Physical vapor deposition runs between 200 and 550 °C (400 and 1025 °F), which keeps it inside the working range of high speed steel. The table on this page is measured tool life for coated and uncoated high speed steel tools, tool by tool and workpiece material by workpiece material.
Why the tempering temperature decides the process
A coating only helps if the steel underneath it keeps the hardness the tool was bought for. CVD is a high temperature process, so on any tool steel it sits above the tempering temperature, and the tool has to be tempered a second time after the coating is applied. That second temper has to be chosen so that it does not soften the substrate while it is conditioning the coating. PVD works the other way. It deposits at temperatures low enough that a fully hardened high speed steel tool keeps its hardness through the cycle, which is why PVD became the default coating route for mills, drills, taps and hobs.
Chemical vapor deposition
CVD is carried out in a vacuum chamber and builds the coating from a gas that reacts at the tool surface. Chromium, aluminum oxide, titanium carbide, chromium carbide, iron nitride and titanium nitride have all been used as coating materials. The process temperature is usually above 800 °C (1472 °F). TiC and TiN are the two wear resistant coatings in common use on high speed, cold work and hot work tool steels, and the coating thickness they are deposited at runs from 2 to 20 µm (0.0001 to 0.001 in.). The wear mechanism that a CVD coating defeats is abrasion, through its hardness. The source adds one process caution. The chlorine content of the coating has to be held below 5%, because above that level the wear resistance of the coating degrades.
The tool steels the source lists as successfully CVD coated are the AISI A, S, D, H, M and T types. The lower alloyed S grades and all of the W and O grades are difficult or impossible to coat properly, because their austenitizing temperatures are too low for the process.
Physical vapor deposition
PVD also works in a vacuum chamber, and it relies on plasma assisted precipitation of TiC or TiN onto the tool steel at 200 to 550 °C (400 to 1025 °F). That range is what makes PVD the coating route for high speed steel, since the CVD temperature would take the tool back below its working hardness. Tool steel wear is reduced in about the same proportion, 2 to 6 times less wear, whether TiC or TiN is applied by CVD or by PVD, so the process choice is made on temperature and on cost rather than on wear resistance.
Two measured results in the source show how much of the gain is friction rather than hardness. TiN coatings on H13 pins reduced the friction coefficient in pin on disk tests from 0.7 to less than 0.2. A modified ASTM G65-10 abrasive wear test on D3 tool steel found that wear on the TiN coated samples was between 4 and 23% of the wear on the uncoated samples, depending on the initial surface roughness of the specimen.
The surface roughness condition on a coated tool
The same work led Sundquist and coworkers to a condition that shops miss. A TiN coating can only add tool life when the surface roughness of the tool is finer than the coating thickness. A tool ground to scratches deeper than the coating carries the coating on the crests of the grinding marks with the valleys bare, and the coating is worn or knocked off the crests first. Grinding and polishing the tool to a finer finish before coating is what converts the coating into tool life, and it is the reason two identical tools from two shops can show very different gains from the same coating.
Tool life of PVD coated cutting tools
Table 5 gives workpieces machined before resharpening for a set of high speed steel tools, each one run uncoated and then coated. The gain is largest where the workpiece material is hardest to cut. An M7 end mill cutting 6061-T6 aluminum went from 166 pieces to 1500, a tap cutting 1050 steel at 30 to 33 HRC went from 60 to 70 pieces to 750 to 800 pieces, and a broach insert cutting type 303 stainless steel went from 100,000 pieces to 300,000. Two tools in the table carry a coating other than TiN. The T15 form tool cutting 1045 steel is coated with TiC, and the M2 cutoff tool cutting low carbon steel is coated with a TiC-TiN combination.
| Cutting tool | High speed tool steel, AISI type | Coating | Workpiece material | Workpieces machined before resharpening | |
|---|---|---|---|---|---|
| Uncoated | Coated | ||||
| End mill | M7 | TiN | 1022 steel, 35 HRC | 325 | 1200 |
| End mill | M7 | TiN | 6061-T6 aluminum alloy | 166 | 1500 |
| End mill | M3 | TiN | 7075T aluminum alloy | 9 | 53 |
| Gear hob | M2 | TiN | 8620 steel | 40 | 80 |
| Broach insert | M3 | TiN | Type 303 stainless steel | 100,000 | 300,000 |
| Broach | M2 | TiN | 48% nickel alloy | 200 | 3400 |
| Broach | M2 | TiN | Type 410 stainless steel | 10,000-12,000 | 31,000 |
| Pipe tap | M2 | TiN | Gray iron | 3000 | 9000 |
| Tap | M2 | TiN | 1050 steel, 30-33 HRC | 60-70 | 750-800 |
| Form tool | T15 | TiC | 1045 steel | 5000 | 23,000 |
| Form tool | T15 | TiN | Type 303 stainless steel | 1840 | 5890 |
| Cutoff tool | M2 | TiC-TiN | Low-carbon steel | 150 | 1000 |
| Drill | M7 | TiN | Low-carbon steel | 1000 | 4000 |
| Drill | M7 | TiN | Titanium alloy 662 layered with D6AC tool steel, 48-50 HRC | 9 | 86 |
Source, Table 5 of Surface Engineering of Specialty Steels, in ASM Handbook Volume 5, Surface Engineering.
What the numbers assume
Every row is the practice of the shop or laboratory that supplied it to the source, so workpieces per tool is a reference point and not a guarantee. Cutting speed, feed, coolant, machine rigidity and the condition of the workpiece material all move the count. A coated tool also has one cost that an uncoated tool does not carry. Each regrind takes the coating off the cutting edge, so the tool has to be recoated to keep the life gain, and that has to be priced into the tool cost before the switch is made. Use the table as evidence that coating pays on the difficult workpiece materials, then set the cutting data on a first article.
Source: ASM Handbook, Volume 5, Surface Engineering, ASM International, 1994.