We have just started a new project. It’s for the European Space Agency and we are part of a group designing a new engine for missions to Mars and exciting stuff like that. Interesting stuff.
Our particular brief is to look at high-temperature materials an amazing number of which are made by CVD or CVI: For instance, W, Re, Ir, Nb, Ta, SiC, HfC & TaC
Hafnium carbide is the most refractory binary compound known to man with a melting point of 3890degC. 480 degrees higher than tungsten! The mixed carbide tantalum hafnium carbide has an even higher melting point of 4215degC. Considering the surface of the sun is ‘only’ 5500degC or so, that’s pretty mind-blowing!
Just like its better known fellow Group4a elements, titanium and zirconium, it can (of course) be deposited by CVD by the reaction of hafnium tetrachloride with methane in the presence of hydrogen.
As a CVD coating HfC, just like its close relative, titanium carbide, TiC, is used on cutting tools, but more interestingly it is also used as a barrier layer to protect the fibres in carbon fibre reinforced carbon composites (CFCs) from oxidation at high temperatures. At first sight this seems pretty odd because HfC by itself has a poor oxidation resistance, but in combination with a carbon underlayer and a silicon carbide over-layer (both also deposited by CVD) a great protective thin film of mixed oxides is formed.
AFAIK nobody has tried to deposit Ta4HfC5 by CVD. Perhaps I should try it!
Free-standing CVD W shapes
I love tungsten! At 3410 degrees C, it has the highest melting point of all metals. This makes it an incredibly useful metal, even if the green lobby manage to get rid of all the ordinary light bulbs out there. Electrical contacts, x-ray targets, windings & heating elements, space, missile, tools and any number of high temperature applications.
Tungsten’s very high melting point is also its weakness: It makes it very hard to work: Melting & casting is practically impossible, sawing, machining & forming very difficult.
Luckily tungsten is easily deposited by CVD, normally by the reaction of tungsten hexafluoride with hydrogen. This can be achieved at the modest temperature of 500 degrees C. Many different materials can be coated with a layer of tungsten from a micron to a few millimeters thick.
The relatively low deposition temperature means that steels can be coated (with a suitable interlayer) without distortion or the need for post-coating re-hardening.
By depositing a thick coating onto a removable/dissolvable/meltable/burnable mould, free-standing CVD tungsten shapes can be created. Thin-walled, boats, tubes, crucibles in 100% dense, high purity tungsten can be made. This is practically impossible by any other route.
If wou want to know more check out www.cvd.co.uk/othercoatings.html
Welcome
Welcome to my first ever blog!
For the uninitiated CVD in this instance stands for Chemical Vapour Deposition as opposed to Cardio Vascular Disease (or Clyde Valley Drilling or Commercial Vehicles Direct or even Charterhouse Voice and Data).
If you Google ‘CVD’ you’ll soon find the Wiki for ‘Chemical Vapor Deposition’ obvious written by an American friend: Why DO they insist on changing perfectly good spellings?? Nonetheless an accurate piece even if it was written by an expert from the semiconductor industry with an obsession for acronyms. There are however many fields other than semiconductors where CVD finds applications. Nuclear, machine tools, automotive, oils & gas, aerospace, solar power, glass, electronic, medical and military to name but a few. Somewhere at the bottom of page 1 of your ‘CVD Google’ you will find www.cvd.co.uk . This is the website of my Company, Archer Technicoat Ltd, ATL. Based in the UK, we specialise in the process of CVD and its application across as wide a spectrum of industries as possible.
What I hope to achieve with this blog is to regularly mention a particular coating or application, where hopefully a colleague in a different sector might think “I never knew you could do that with CVD” or a non-specialist might think ” Wow, that’s a pretty cool thing”.
I’ve got a few things in mind to kick off with but no doubt as time passes, things will get harder, so if something occurs to you, please feel free to contribute!
Cheers
John
