Tungsten carbide coating: the layers on carbide, and carbide as the layer
A tungsten carbide coating is either a thin hard film put on a carbide tool or a layer of tungsten carbide sprayed or welded onto steel. Both are covered below; to sell coated inserts, send us photos.
What does tungsten carbide coating mean?
The phrase names two opposite jobs. One is a film such as titanium nitride (TiN) on a cemented carbide tool: grains of tungsten carbide (WC) held by a cobalt binder (see grades and binders). The other is tungsten carbide itself, put onto steel as a wear surface.
In ITIA's 2010 overview, more than 80% of turning inserts and about 70% of milling inserts were coated; an insert is the small replaceable cutting tip clamped into a turning or milling holder.
Coating began in the early 1960s with TiC and TiN; ITIA's 2010 list adds Ti(C,N), aluminium oxide (Al2O3), TiB2, TiAlN, AlCrN, diamond and diamond-like carbon.
What does a coating do for a carbide insert?
A coating is there to give the cutting edge a hard, slick, heat-stable face. ITIA lists the main requirements as high hardness, high wear resistance, low friction, high thermal stability and high oxidation resistance.
The carbide under the film still carries the load. ITIA says the substrate must give the coating rigidity, creep resistance, toughness and thermal support, and bond well enough that the layer does not spall (flake away).
CVD vs PVD coating: how the two processes compare
CVD (chemical vapour deposition) runs hot and PVD (physical vapour deposition) runs far cooler, and several differences below follow from it.
| CVD | PVD | |
|---|---|---|
| Process temperature | 800-1150 °C substrate, thermally activated CVD (2022 review); 1100 °C in a 2023 paper | About 200-500 °C (2023 paper) |
| Coating thickness | 5-20 µm (ITIA, 2010) | 2-8 µm (ITIA, 2010) |
| Effect on the carbide | Anneals the tool; a brittle eta phase (Co-W-C) can form in the WC-Co, and deposition at 1100 °C reduces tool strength (2023) | The low heat allows multilayer coatings (2023) |
| Shapes and batches | Coats complicated shapes well, in large batches (2022) | Line of sight, so very complex shapes may not suit it (2022) |
| Stress and edge | Often tensile residual stress (2022) | Gives the keenest edges (ITIA) |
| Suited to | High coating thicknesses (2022) | Mostly finishing, as the layer is thinner (2022) |
| Example in the sources | Diamond layers, by plasma-activated CVD (ITIA) | TiAlN on a submicron WC-Co grade with 5-8 wt.% cobalt, for machining aluminium wheels (ITIA) |
Even the thickest CVD figure, 20 µm, is 0.02 mm, under a thousandth of an inch (about 0.0008 in).
Many inserts carry a stack rather than one film: ITIA describes a coated turning insert with thin layers of Ti(C,N), alumina and TiN.
Sources: ITIA, Cemented carbides (2010 newsletter); Coatings review, Materials (2022); Milling tool coatings paper (2023), OSTI.
Carbide insert coatings by colour
Gold means TiN in all three sources. The other colours come from a university PVD lab in Estonia (TalTech) and Slovenia's national research institute (JSI).
| Coating | Colour | Hardness | Max. operating temperature |
|---|---|---|---|
| TiN | Gold (TalTech, JSI, ITIA) | 24 GPa (TalTech); 2300 HV (JSI) | 600 °C (TalTech); 500 °C (JSI) |
| TiCN | Copper (TalTech) | 32 GPa | 400 °C |
| TiAlN | Lilac-black (TalTech); dark violet (JSI) | 35 GPa single layer, 28 GPa multilayer (TalTech); 3300 HV (JSI) | 800 / 700 °C (TalTech); 900 °C (JSI) |
| AlTiN | Black (TalTech) | 36 GPa | 900 °C |
| CrN | Silver (TalTech); metallic grey (JSI) | 18 GPa (TalTech); 1800 HV (JSI) | 700 °C (TalTech); 750 °C (JSI) |
| AlCrN | Bluish gray (TalTech) | 32 GPa | 1000 °C |
TalTech gives microhardness in GPa and JSI gives Vickers hardness (HV, resistance to a diamond indenter); the lists disagree in places, so read colour as a clue, not proof.
The aluminium-rich nitrides take more heat. A 2023 paper puts (Al,Ti)N-coated tools at about 2720 HV with an oxidation temperature near 840 °C, against about 1930 HV and 620 °C for TiN, and reports less flank wear for the (Al,Ti)N tools milling steel at 600 m/min.
Diamond is the exception on abrasive alloys. The same paper says variations of conventional coatings do not improve tool life on hypereutectic aluminium-silicon alloys, but diamond coatings do.
Sources: ITIA, Cemented carbides (2010 newsletter); TalTech thin coatings lab, PVD coatings; Jozef Stefan Institute, hard coatings list; Milling tool coatings paper (2023), OSTI; NTP hard metals background document (2010).
Tungsten carbide as a coating: HVOF thermal spray
When tungsten carbide is the coating, it is often sprayed on by HVOF (high-velocity oxygen-fuel). A 2006 SERDP/ESTCP summary describes powder injected into a supersonic flame of hydrogen, propylene or kerosene, where the particles soften and strike the part fast enough to form a dense, well-adhered layer.
The powders named are WC-Co (carbide in cobalt) or WC-CoCr (carbide in cobalt-chromium). A 2004 Journal of Thermal Spray Technology paper says HVOF layers show fewer phase changes and lower porosity than other spray methods, and SERDP (2005) judged conventional plasma-sprayed WC/Co too porous to replace hard chrome directly.
Sprayed carbide is thick by coating standards. SERDP/ESTCP gives 0.003 inch (about 76 µm) on new parts and up to 0.015 inch (about 381 µm) to rebuild worn ones, the same range as chrome plating, and a 2022 review says thermal spray can reach several millimetres. Even 0.003 inch is nearly four times the thickest CVD film above.
Sources: SERDP/ESTCP WP-200023 (completed 2006); J. Thermal Spray Technology (2004); SERDP WP-1151 (completed 2005); Coatings review, Materials (2022); Sandia report SAND2017-0715J, OSTI.
Sprayed WC-Co and WC-CoCr in place of hard chrome
Sprayed carbide beat hard chrome on the measures below, and hard chrome plating depends on hexavalent chromium, a known carcinogen. The 2006 SERDP/ESTCP summary notes that OSHA had lowered the workplace limit (PEL) for it to 5 µg/m³, from 52 µg/m³.
A SERDP summary (completed 2005) says a US/Canadian team was validating HVOF WC/Co and WC/CoCr for outer diameters and external surfaces, which SERDP put at about 75 percent of Department of Defense hard chrome applications; HVOF guns then could coat only inside diameters over 11 inches.
- SERDP (2005): in general, WC/Co coatings last at least two to three times longer than hard chrome.
- The US Navy chose HVOF WC-17Co to replace electroplated hard chrome. A P-3 main landing gear strut with an HVOF WC-Co coating made 6,378 landings from April 1999 to January 2006, with none of the edge chipping typical of chrome on its axle journals; shallow pits on the barrel were likely seawater attacking the cobalt.
- A 2018 hydraulic-rod study on AISI 1045 steel measured HVOF WC-CoCr at about 1256 HV0.01 against 952 HV0.01 for hard chrome (a 10 g load), with layers of 167 µm and 101 µm.
- In a 2015 university study, sprayed WC-10Co4Cr 250-300 µm thick clearly outperformed electrolytic hard chrome in room-temperature sliding wear against alumina.
Sources: SERDP/ESTCP WP-200023 (completed 2006); SERDP WP-1151 (completed 2005); NASA NTRS, Navy HVOF WC-Co service evaluation; HVOF WC-CoCr vs hard chrome on hydraulic rods (2018); Tribology of HVOF and HVAF WC-10Co4Cr (2015).
Hardfacing: tungsten carbide welded onto steel
A 1943 US government hard-surfacing guide defines the job as laying a hard, wear-resistant alloy onto a metal wearing surface by welding. That guide already gave crushed cast tungsten carbide in a high-strength steel binder a rod class of its own, Class IV.
Fused tungsten carbide, a melt cast and quenched fast into very hard fine-crystal particles, is used for surface hardening (ITIA). A 2023 paper finds macrocrystalline WC less prone to dissolve than cast WC/W2C, and names WC-Co as the most common strengthening phase in hardfacing today.
Tungsten-containing hardfacing protects drill bits and chisels (2021 review), and on mining and construction tools it guards the steel holder of the carbide tip (ITIA).
Sources: Hard surfacing guide (1943), GovInfo; ITIA, Properties of intermediates; Hardfacing overlays, Materials Science-Poland (2023); Tungsten recycling review (2021), ITIA-hosted; ITIA, Cemented carbides (2010 newsletter).
Coated inserts are still carbide underneath
A coated insert is a carbide insert wearing a film of 2-20 µm (ITIA). ISO 513:2012 builds this into its letter codes: HW is uncoated carbide, mainly WC, and HC is any of its carbide classes with a coating.
The film is a recycler's problem, not yours. Coatings and solder have to be removed before carbide goes through the zinc process, which does not purify the powder, or they end up in the recycled material (USGS 2001; ITIA 2019).
Leave the coating on: we buy carbide in any form, gold, black or bare, sharp or chipped. Send photos and a rough weight through the quote form, then ship to us from any state (we pay the shipping) or ask for pickup in California, Arizona or Nevada; see how selling works.
Sources: ISO 513:2012 preview; ITIA, Cemented carbides (2010 newsletter); USGS Open-File Report 01-349; ITIA recycling newsletter (2019).
Questions people ask
CVD vs PVD coating: what is the real difference?
Heat and thickness. CVD runs hot and builds the thicker films; PVD runs far cooler, gives the keenest edges and is mostly used for finishing work.
Do carbide insert coatings contain tungsten?
The ones named here do not. TiN, TiCN, TiAlN, AlCrN, alumina and diamond have no tungsten in their formulas; the tungsten sits in the carbide body underneath.
Can I identify a carbide insert coating by its colour?
Only roughly. Gold is TiN in every list above, but the two lab lists disagree on TiAlN (lilac-black against dark violet) and on CrN (silver against metallic grey).
Should I strip the coating before selling carbide scrap?
No. Removing coatings is a recycling step, and we buy carbide in any form, coated or not. Photos and a rough weight on the quote form are enough to start.
Have tungsten carbide to sell?
Tell us what form it is in and about how much it weighs, and add a few photos. Ship it from any state and we pay the shipping, or we pick up anywhere in California, Arizona and Nevada.