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
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TaCl5 Generator
Tantalum is an extraordinary metal. It has a melting point of 3017degC which makes it the third highest metal behind Tungsten and Rhenium. This is a useful property, however it is its extreme chemical resistance that makes it most interesting. Below 150degC it is inert to aqua regia (mixture of concentrated hydrochloric and nitric acids). Aqua regia dissolves other metals, such as gold and platinum, normally thought of as inert, in a trice.
Tantalum has a variety of uses, the biggest of which is in capacitors, but for coatings, the interest in mainly in putting an extremely corrosion resistant layer onto fasteners, crucibles, pipe bores etc. In the last year the price of tantalum metal has rocketed and this is expected to stimulate interest in coatings where solid tantalum components could be replaced by cheaper metals coated with tantalum.
Tantalum is fairly easy to deposit by CVD, normally from TaCl5 & hydrogen. The main difficulty is that TaCl5 is practically impossible to evaporate in a controlled manner. TaCl5 melts at 216degC and evaporates at around 240degC The change from liquid to vapour involves dissociation of the dimer Ta2Cl10 to the monomer, so this complicates things. It is also hygroscopic so that any exposure to air completely changes the evaporation properties. ATL overcomes this problem by generating the tantalum in situ through the reaction of chlorine gas with tantalum metal. Obviously handling a gas as toxic as chlorine is not to be taken lightly, but it does make Ta CVD a reliable, repeatable and scalable process. Chlorine is cheap and tantalum scrap is relatively easy to buy, although prices are currently eye-watering (I blame the Chinese).
ATL has developed and sell its own design of industrial scale TaCl5 generator. Transport of up to 1Kg per hour of Ta metal is possible with the current design. These are available for sale either as part of a complete turn-key coating system or as a bolt on to customers’ existing CVD equipment. Control can be stand alone or incorporated into the main system. The same generators can be used for making TaC coatings which are of increasing interest in the semi-conductor market.
Silicon Carbide deposited by CVD is a wonderfully versatile material with a myriad of applications and excellent physical properties.
Its high hardness means it has abrasive and protective applications.
Its 100% dense structure combined with excellent chemical resistance gives it corrosion resistance applications.
Its high temperature strength combined with low density (3.2 g/cc) makes it an ideal space, rocketry and aerospace material.
These properties plus its low neutron activation cross-section makes it an ideal material for fission and fusion applications.
All those tiles on the underside, leading wing edges and nose cone of the Space Shuttle may have got bad press, but they are SiC and there’s no better material.
All silicon chips have passed across CVD SiC Coated furniture during their manufacture. The SiC is dense, pure and inert enough to keep the required incredibly high purity levels. For the most critical semiconductor applications, the furniture is made from solid CVD SiC. This is possible because SiC is one of those materials (like tungsten) that can be deposited several cm thick by CVD.
I could go on an on, but then I really would be rambling!
ATL Presentation
So now you know all about CVD: It’s the process for you and you are looking for a CVD specialist. Look no further!
Make sure you’ve got your sound switched on.
Apologies for the blatant self-publicity, but it is a pretty nifty presentation.
A rather nifty presentation on CVD, how it works, its applications and advantages.
Make sure you’ve got your sound turned on!
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
