Way outside the norm - Alumina coating

[tellner]

I know this is lightyears away from what usually gets talked about, but does anyone here have experience with thin film or thicker alumina coatings - theory, methods (PVD, CVD, sputtering, etc.), requirements, doping and so on? I've got the cube root of bother-all experience in this and don't know where to start reading.

Thanks in advance

 

[RandomPhantom700]

Does wrapping food in aluminum foil count? :shrug:

 

[DavidCC]

I just had my car spray-welded for rust repair, is that close? ("Rust-busters")

 

[Ninjamom]

Can you give any details at all about your intended application?

In general the best place I've found for information about way-off-topic subjects like this is to find a good review article in the proceedings from some relevant symposia. The organizers of such symposia usually pick some 'big name' in the field to give an invited paper, and often as not, it ends up being a review of the current state of the art.

 

[tellner]

There are two possible applications at the moment, ninjamom.

The first is a window coating. I saw a couple ads for monitors which were alumina-coated. They stood up nicely to scratches, hammers and crossbow bolts. It's worth a bit of extra money because of what's behind the windows.

The second is doped with things like chromium and other coloring agents. I'd like to build up (scratch-resistant, high tensile strength) sheets as tiles from 1x1 inch possibly up to 4x4.

I've had a little experience with flux-fusion synthetic gemstone creation. It was strictly proof-of-concept and by the book where the book was almost 100 years old. I know that alumina and synthetic corundum stone films are down to a routine industrial process and want to look into contracting out or doing prototype pieces with my own modified equipment.

 

[Ninjamom]

Dang, Tellner! I have to admit, I don't know many people with a spare PVD chamber just sitting around the garage waiting for work like this

I haven't a clue what you might want the tiles for, but I have an idea for those. I came across an article about research in ways to make armored optical barriers (i.e., windows for VIP vehicles in hostile areas, helmet visors for the explosive ordnance disposal guys, and such). These, of course, have to withstand explosions and abrasions beyond what your monitor is likely to face. Anyway, there is a section in the article about spinels (which are really just doped aluminum oxides) and single crystal sapphire (which is also an aluminum oxide). While it might sound expensive, they have already gotten single-crystal boules to over a foot across (so, you could cut those to form tiles larger than what you need), and they have some other process that produces single crystal sheets up to 12" by 15". Check out the relevant section in this article, then use this link to DTIC to find more information about keywords and processes you find of interest. I have read of the same process being used with garnets and related materials, as well. (Aside: DTIC is the "Defense Technical Information Center", a repository for all kinds of federally-funded research, much of which is available free online to the general public).

About the coatings: from what you're saying, I *think* you're looking at hardened coatings for protection, coupled with or integrated into a quarter-wave AR optical stack for glare/reflection reduction. Am I close? If so, you might use whatever software you have for designing your coating, and see if you can optimize performance, assuming you have a thicker layer or two of your chosen alumina-material incorporated into the stack. (If you don't have such a computer program, GET ONE!!) You may also need to do some library-work on comparing several different kinds of crystalline structure in your stack. It is possible to get films or layers that are single-crystal, polycrystalline, or amorphous (non-crystalline). Generally speaking, the better the crystalline structure, the more difficult/expensive the film is to produce, BUT the optical properties tend to improve. If all you need do is look through the window (i.e., read a monitor, as opposed to pass a laser through it or have some exotic sensor peer through it), then you might do with the less-expensive approach.

About the only book I can offer as an overview on coating technology is this one. It was designed as a primer on 'things-to-know-and-think-about-when-you-say-you-need-an-optical-coating-made,-and-you-wonder-why-its-so-darn-expensive' for government techno geeks. It describes the basic physics of thin film optical coatings and why they work, the bare-bones basics of some different deposition processes, how to measure their optical performance, different types of filter designs, different types of substrate materials, and some advanced coating methods. The appendices include government Mil-Spec tests for optical coatings, and a BASIC program for predicting thin film performance (in case you don't already have one). The information is somewhat dated, but then, so am I (I have a copy of the original 1987 ed., no longer available.) At least it should provide you with a good starting point, and get you familiar with the key words and phrases in 'coating-speak'.

Hope this helps.