Tuesday, February 16, 2010

Etching zirconium oxide with hydrofluoric acid

I am currently working on a project where I need to replace a standard mouse ball with a non-metallic equivalent. Normal mouse balls are built from a steel core that has been coated with a thick layer of rubber. I can't use any metal in my project, so I must find a replacement. The main requirements are:

Ball must be very dense
Ball must be non-magnetic and non-conductive (ie non-metallic)
Ball must be coated with rubber or naturally have a high friction coefficient
Outer diameter 7/8"

My first solution was to use 7/8" glass marbles. These are not easy to find, but they are available. The problem is that the marbles are not perfectly spherical, and they are not nearly as dense as the original mouse ball. The lower density means the ball weighs less and will not track smoothly on the mousepad surface, since it sometimes skips and hops. I was able to etch them with hydrofluoric acid, spray them with primer paint, then coat with with spray-on rubber. They worked OK, but I have a better solution now.

I found a source of 7/8" dia zirconium oxide balls (http://www.ortechceramics.com/). This stuff is a ceramic, but 2-3 times as dense as glass. It's non-metallic, and the balls have very high sphericity. The problem is that the balls come very highly polished, and the paint will not stick to them. I tried etching them with standard glass etching cream, but there was no effect.

I found that a local lab supply company (http://www.lab-proinc.com/) sells 50% hydrofluoric acid (HF). I bought some and initially made a small 10% dilution. The ZrO2 showed no etching after 1 hour of full submersion in the dilution. I upped the concentration to 20%. Still no effect after 30 minutes. Finally, I submerged the ZrO2 in straight 50% HF. After 10 minutes, I could see the shine starting to disappear from the balls' surfaces. After 20-30 minutes, the balls appeared to have enough etching so that they would hold paint well.

Hydrofluoric acid is scary stuff. There are reports of it penetrating skin painlessly then dissolving bones and causing blood toxicity. Standard nitrile or latex gloves only provide splash protection -- ie if the HF is spilled on the glove, immediately remove the glove, and get a new one. HF will penetrate the gloves if left in contact for too long.

I was surprised to see the 50% HF was actually 'fuming'. When I left it in the open container, I could clearly see vapor droplets coming off the surface. If I blew across the surface of the HF, more vapors were produced. I am not sure if it is reacting with water in the air/breath or what. I didn't expect 50% to be fuming.


  1. Well I was thaught that zirconium oxide couldn't be etched in HF as it was not in fact ceramic as we know it. I think that the circonium balls have been covered with ceramic glaze that is why you claim they were polished and that is why you could etch them.

  2. You put yourself in great danger by using HF. You easily could have killed yourself, it's not just "scary stuff", and there haven't been just "reports", this is one of the most dangerous chemicals known.

  3. Anonymous, I read a lot of hobbyist and experimenter blogs, and it never fails that someone leaves a comment describing how dangerous the process is. If you have personal experience, or know personally of someone who was injured by the chemical, please share the information here. Otherwise, your post is just another "report" of how dangerous this chemical is.

    Since there is no explosion risk, and the 50% HF can be diluted further without much heat being released, and the fuming properties are pretty mild, I would argue the risk is actually lower than many other lab chemicals. Just don't let it spill on skin, and everything will be fine.

  4. even really low concentrations (.5%) are extremely dangerous. when we work with it we use splash guards, facemask, etc. use two pairs of gloves and just be careful.

    although you should be able to dissolve zirconia in heated HF with lower concentrations.

  5. When using HF to etch silicon dioxide in microelectronics fabrication, we wear full length splash smocks, face shield, super thick gloves in a laminar flow chemical hood. If any splashes on open skin, immediate application of calcium gluconate to the area is essential and we were required to go to the ER for further care and observation. If I remember correctly, HF works by attacking the bone and preferently replacing the calcium.


  6. Andrew, I know that HF acid is a very dangerous substance. However, I feel more comfortable working with it than I would with explosive, fuming, or highly reactive substances as I mentioned above. %50 HF acid does not fume very much so in rooms with normal ventilation, splash protection is all that is needed to be safe. There are many other chemicals that produce poisonous gas in great quantity, which should be classified as generally more dangerous than HF acid. Have you witnessed or experienced any HF accidents yourself?

  7. luckily I have not but my coworkers have. I'll just classify you as a brave soul, or maybe I have just succumbed to scare tactics.

  8. You could have just put a ball (or all of them) in a container of a course de-burring material or powder that was harder than the zirconium oxide and rolled it around for a few hours to wear away / scratch a bit of the surface. Ball mill or vibratory mill / tumbler... or just shaking it in a bottle by hand.

    Silicon carbide and Corundum (Aluminum oxide) are both harder than zirconium oxide and available at ceramic chemical suppliers among other places. You could probably even use a sanding sponge from a hardware store since they mostly use aluminum oxide and silicon carbide as the abrasive. The scratches don't have to be perfectly uniform so long as the rubber will grip it.

    You can completely avoid the unnecessary danger and expense of dealing with HF. With most common acids if you get a little on your skin you have a few minutes to wash it off before it breaks the skin, and even if it does, its just a little wound, it may be painful but your life is not usually in real danger. With HF, you get some on you and you and you treat it with calcium gluconate and go to the ER immediately or risk permanent bone / nerve damage... That should put it into perspective.

  9. Anonymous, as I mentioned above, either post a link to a factual, verifiable account of an HF accident or don't post about its hazards at all. There is too much chatter on the internet about HF, and too little actual information.

    Using SiC sandpaper to scratch the surface of the ZnO balls would not work. Using a rotary or vibratory tumbler might, but the media would need to have extremely sharp points to make scratches, and would not last long at all. It sounds like you haven't tried either method.

  10. Google images, "hydrofluoric acid burn"

    It doesn't have to make deep scratches, it just has to make the surface less "perfect." If the rubber sticks enough to coat the whole ball, then your good. You could even manually scribe the balls with a diamond/SiC tipped ____. So long as the rubber sticks the created imperfections will be filled in.

  11. I know you may think hydroflouric acid is just another ordinary chemical, but its not. First of all, just having enough touch your skin can cause you to go into cardiac arrest. If you want to take the chances, thats not my problem.

  12. Ben, I know this is a little after the fact but I wanted to respond to your queries of actual HF incidents. I work at a petroleum refinery and we use 100% HF acid in some of our processes. I think you have the right idea of HF being very dangerous but hopefully I can explain why it is so dangerous. First off, HF loves calcium and other elements (potassium, magnesium, etc.). When it comes into contact with human skin, it will start to penetrate slowly and seek calcium wherever it can find it. The longer HF is left untreated the more damage it does. Eventually it gets down to the blood level and will start to extract calcium from the bloodstream. This is when it becomes life threatening because the heart relies on different elements in the blood to create the rhythmic pulsing and when those levels of different elements (mainly calcium) are disturbed it can send the victim into cardiac arrest. I personally know a person that was exposed to HF acid and he luckily survived because he was treated very quickly but the pain is intense when you encounter the stuff at higher concentrations.

    The main reason lower concentrations (such as 50%) can be very dangerous is because when you get acid on your skin you probably won't feel any burning effects until at least a few hours after the contact and by that time it is pretty late in the game to treat but can be treated. I have personally worked around both the diluted (10% or so) and the concentrated (90%) and the major difference is that you can feel the exposure immediately at higher concentrations. I have never had to be treated (each time I got a slight whif it was in minute amounts) but I know that it can be very dangerous because I have read many articles and talked with many people concerning this substance. I also provide training courses on it so that people know to respect it and how they can protect themselves when they work around it.

    If I could leave you with any advice, it would be to protect yourself heavily if you ever do any work with HF in the future. At least wear safety goggles and a face shield, a full protective suit, neoprene gloves and an air-purifying respirator (not just a dust mask). Sorry for the lengthy post but I hate to see anyone get hurt around this stuff. Best of luck in your future endeavors!

  13. Good work.. but since you asked in your comment...
    we regularly use HF to etch Titanium, Zirconia etc.... once my college had a hole in his glove and a lil HF stuck to his finger nail. It wasn't serious but he had to get his pointing finger nail removed for stopping continued burning caused by it staying under the nails. Then we used a overnight spraying of antidote calcium gluconate on the bare skin.

  14. I'm preparing to work with this in my lab and I was hoping you might comment on where you found containers suitable for putting the solution in?

  15. Boron Trifloride