Apple Invents Coating Method for Scratch Resistant Stainless Steel

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A patent application filed by Apple shows the company is looking for ways to manufacture stainless steel with an invisible coating that would make it both scratch and impact resistant. The patent application shows that the company has invented a way of doing so by adding a nitride layer on top of the metal that would protect the metal, even while preserving the “natural surface color and texture of stainless steel.”

In the unusually easy-to-understand and clear abstract for the application, Apple characterized its invention as, “a cost effective system, method and apparatus adapted to provide a nitride layer on stainless steel used for the manufacture of consumer electronic products. In addition to providing a durable, hard surface that is both scratch and impact resistant, the nitride layer allows for the natural surface color and texture of the underlying stainless steel to remain visible to the user. It is this natural surface color and texture of the stainless steel that adds to the aesthetically pleasing appearance of the consumer electronic product thereby enhancing the user’s overall experience.”

In other words, Apple could use this system to make Macs or iOS devices with stainless steel covers without having to put a coating on the metal that changed its appearance.

The patent goes on to describe exactly how Apple would achieve this using a process of nitrogen based salt baths of “no more than 580° C” to give the stainless steel a nitride layer. The resulting layer would be from 15 to 30 .µ (microns) thick, and would give the treated metal a Vickers Hardness Value of 1,000.

And this is where we risk getting in over our heads, because that number needs context to understand its true meaning. Without getting into the boring details of the Vickers Hardness Test, the short version is that it’s a method for testing how hard a metal is by measuring its resistance to being deformed by a standard object.

It is usually express as a hardness value accompanied by the load of the object used to test, as in 140HV30. That’s the Vickers Hardness Value of Stainless Steel 316L, a standard form of stainless steel, and it happens to be the specific kind of stainless steel discussed by Apple in this patent application.

Apple is saying that its invention will take stainless steel 316L from 140HV30 to “at least approximately” 1,000HV30, a 7.14x improvement. And now we can all be suitably impressed.

The following image was provided by Apple in its application and more or less shows how the nitride layer would be applied, one molecule at a time.

Nitride Patent Application Figure

The inventor of the process is Douglas Weber, and the application was filed on April 6th, 2010 and published on October 28th, 2010. The good people at PatentlyApple were the first to spot it. The application is titled, “Nitriding Stainless Steel for Consumer Electronic Products.”

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13 Comments Leave Your Own

Bryan Chaffin

I’d love it if any science nerds could correct any errors I might have made in this piece.  Thanks in advance. smile

In the meanwhile, this seems like a great invention to me, and one that could really give Apple a leg up on the competition in making stronger devices that were more resistant to damage. Lower replacement costs, higher perceived value, better appearance…these are all big wins for Apple shareholders and customers alike.

VaughnSC

15 to 30 .mu.m thick (microns)

Bryan: μ (lowercase Greek letter mu) is the symbol for microns, as I recall; just type ‘μ’ in the text. My tiny contribution.

xmattingly

Would have been nice if Apple had gotten on the ball with this sooner, since they insist on using scratch-a-riffic stainless steel on their iPod Touches.

Bryan Chaffin

My tiny contribution.

...“tiny contribution”... HAHAHAHAHA!!!

Very nice, VaughnSC, and thanks for the info. I edited the story to use the correct symbol.

Thanks!

Lee Dronick

So this would be SCRES?

When I was in the Navy, stainless steel was called CRES for Corrosion Resistant Steel.

I wonder if Apple came up with this in-house or did they buy an idea from someone.

MacKeeper_fan_Mod

I haven’t completely understood from this article if Apple company made this technology for itself or it just buy it from another company. If Apple did it by itself, it is worthy of many accolades. Computer company which spends so lots of money and efforts for conducting such research really takes care on its users.

jfbiii

IDK, if your iDevice has a hardness of 1,000HV30 for more than 4 hours, you should call a doctor.

VaughnSC

you should call a doctor

jbfiii: That was rich ? ‘priAPPism,’ FTW.

other side

In the meanwhile, this seems like a great invention to me, and one that could really give Apple a leg up on the competition in making stronger devices that were more resistant to damage. Lower replacement costs, higher perceived value, better appearance?these are all big wins for Apple shareholders and customers alike.

At least until the coating flakes off.

Stainless is notorious for keeping coatings adhered to.  Esp. in a high-abuse environment like the backside of an iPhone.

Why not just use hardened stainless?

Lee Dronick

At least until the coating flakes off.

Stainless is notorious for keeping coatings adhered to

True that, but the durability of this coating remains to be seen.

“preserving the ?natural surface color and texture of stainless steel.?

Can you stain stainless steel in the manufacturing process? Maybe Apple is looking at colored steel that look like the anodized stuff.

wab95

Reads well, IMO. I see you are using the symbol for microns, courtesy of VaughnSC. Nice touch.

My first thought ‘shields’ or ‘ablative armour’ (I know, Star Trek again).

So this would be SCRES?

My understanding is that this is a bit different than the technology used for CRES. Stainless steel gives rise, so I understand, to a passive chromium oxide coating by way of the higher chromium over carbon content in the steel.

The Apple process is applying a nitride coating to the surface, which does not appear to affect the steel content itself. Then again, I’m a doctor, not a metallurgist.

I think it does signify that we can expect to see more stainless steel in upcoming Apple products (continued iPhone antennae?).

gslusher

At least until the coating flakes off.

As I understand it, there’s no “coating,” like a paint. This forms a nitride layer IN the stainless steel, rather than adding some layer ON the steel. It’s a bit (only a bit! <G>) like anodizing aluminum. To damage the layer, you’d have to remove it by abrasion (e.g., some sandpapers that contain a lot of quartz might work, albeit slowly; this stuff is harder than hardened steel, so most files probably wouldn’t remove it). OTOH, if one were to deform (e.g., bend) the steel enough, the layer might crack, as it’s more brittle than the steel. It’s unlikely that Apple would make a case that could be easily bent/deformed, however, as that would damage a lot more than the nitride layer, like the circuit boards, screen, etc.

This isn’t the first patent relating to nitriding stainless steel. It is a new process, however.

wab95 is correct about how “stainless steel” works. Chromium is actually quite active and readily combines with oxygen. However, the oxide layer formed protects the rest of the steel. In contrast, ferritic iron and steel rusts in air (needs water, but that’s nearly always around) and continues to rust, eventually going all through the metal, as anyone with a beat-up old car knows.

To put this into a different context, here are the Vickers Hardness values of several materials. I show only the HV, as the test load doesn’t matter, as long as it’s enough to dent the surface. See this Wikipedia article and others. These are nominal values, as any material, especially metals, may vary in composition and thus hardness.

Aluminum (pure)  25 (most commercial aluminum alloys are harder)
Gold 35
Copper 40
Iron 30-80
carbon steel 55-120
316L stainless steel 140 (the material referred to in the patent)
limestone 250
window glass 550
granite 850
quartz 1200
tungsten carbide 2500

That puts the nitrided surface in Apple’s patent between granite and quartz! That’s pretty hard. Obviously, this is done AFTER the piece is machined.

FWIW, I’m also not a metallurgist, but I have 3 degrees in mechanical engineering from MIT and once (a long time ago!) taught undergraduate materials science in the US Naval Academy’s Mechanical Engineering Department. I did have to refresh my memory about Vickers hardness testing.

wab95

FWIW, I?m also not a metallurgist, but I have 3 degrees in mechanical engineering from MIT and once (a long time ago!) taught undergraduate materials science in the US Naval Academy?s Mechanical Engineering Department

Many thanks for those clarifications, gslusher. I think 3 degrees in mechanical engineering from MIT qualifies one to have more than an educated opinion.

Great information, and deepens my appreciation for what Apple have done here.

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