Ever drop your phone and wonder why the cover glass survived or broke? Corning did too and in our exclusive tour of their facility in Corning New York, we visited several labs where the company explained all about how the glass breaks, ways to evaluate the mechanisms and the results for its newly developed Gorilla Glass 4 material. Corning has not allowed such access in years, so we were very privileged to learn more and report the details to you.
Corning Overview
First, it should be noted that Corning has been in business since 1861. Glass is obviously a good business to be in – as long as you know how to invent, innovate and listen to your customer’s needs. This is exactly the recipe for success that Sr. VP for Administration and Operations, Charlie Craig explained. Did you know for example, Corning produced some of the first light bulbs for Edison back in 1879? The mass production of light bulbs innovation was then critical for that industry in the 1920s. CRT tubes followed in the 1930s and 1940s with fiber optics being a big breakthrough in the 1970s, followed by flat panel glass. Gorilla Glass was launched in 2006 as a chemically strengthened cover glass for mobile devices.
Craig said that Corning has world class materials scientists working at the Corning HQ facility as well as several other development labs around the globe. The company always works with a lead customer in commercialization projects to develop what he called “keystone components” for these partners that offer “superior performance”. Corning reinvests 8% of its revenue, which is $800M currently, back into R&D. That’s a huge percentage.
The main Corning Sullivan Park campus includes R&D labs where ideas are formulated. Corning has a small glass melting facility where ideas are reduced to glass or ceramic materials for further evaluation. We had a chance to tour this facility and saw the pour out of soda lime glass, which is heated to 1350C. This was poured from a melting crucible into a mold and onto a steel slab. The engineers like to see the material when it is molten like this and as it cools to better assess how pliable it is and how long this malleability lasts. They are also looking for blisters or air pockets. These samples are always annealed afterwards to remove any inherent stress.
Next we saw the same demo but now with Gorilla Glass. This was a lot hotter at 1650C and its malleability period was much shorter than soda lime glass.
Corning also has a pilot production facility so it can start to understand how new materials will be integrated into production. We did not get a tour of this facility.
Gorilla Glass Overview
After lunch, we went over to the Materials Product Performance and Reliability Lab. This is the main area where Gorilla Glass activity is housed – still on the same Sullivan Park campus. This began with a number of slides describing the history of Gorilla Glass and some of the materials science behind it.
Sr. VP and GM for Specialty Materials, Jim Steiner noted that Gorilla Glass was first developed to provide a more rugged cover glass because of breakage concerns with soda lime glass – essentially window glass, which was used to help protect cell phones back in 2005.
In 2006, Corning launched Gorilla Glass with production beginning in 2007. In 2010, it began a branding campaign around Gorilla Glass and in 2012, it introduced Gorilla Glass 2 (GG2), followed by Gorilla Glass 3 (GG3) and Gorilla Glass 4 (GG4) just now. Today, there are over 40 OEM using Gorilla Glass on 1,395 models representing 3 billion devices sold to date.
Gorilla Glass is the result of two unique Corning processes. The first is the fusion forming process that is used for all of its flat panel glass fabrication. This creates a molten pool of glass in a trough that then overflows on both sides. These two thin flows of glass combine under the trough and are pulled down by gravity to form long, high purity glass sheets. This is said to be much better than the conventional “float” process where molten glass rides on a bed of molten tin. This creates glass that has slightly different materials properties on the air and tin sides and requires some finishing before it can be used in flat panel fabs.
The other key technology is the chemical strengthening process. This is a technique whereby Corning replaces larger ions in the glass with smaller ones to create controlled compressive stress on all sides of the glass. This “hardens” the material.
Cover glass on mobile products can be damaged in several ways. Scratches and abrasions are cosmetic issues that degrade the visual quality of the display, but they can also be the precursors to more catastrophic damage which can result from a drop. And, it turns out that the type of surface that the phone falls on makes a big difference. Rough surface like concrete or asphalt are more likely to break the cover glass than smooth surfaces like steel or granite.
How come? Because micro defects in the glass surface are what cause the glass to crack when additional stress is applied. So, a phone that has lots of scratches and abrasions will break easily even when dropped on a smooth surface as those micro-defects act to accelerate breakage. Asphalt and concrete have a rough texture so they are ideal for creating these micro-defects on impact. Then, the force of the impact can cause breakage. – Lab Tours
In the first lab of our tour, we got to see Corning’s fractology testing. Here, Kevin Reiman explained how the company uses its microscopic examination of the cross section of the broken glass to develop a theory for why it failed, including if it failed while in tensile or compressive bending mode. This allows examination of the micro-defects and how these lead to failures. This lab supports the next two labs where damage resistance and drop testing is done.
In the second lab, Corning uses a number of tests to try to recreate the damage that is done in the normal use of a cell phone. For example, phones get scratches and abrasions as the phone rattles around in your pocket or pocketbook. To understand how these defects cause failure, Corning uses procedures like the Abraded Ring-on-Ring and the Scratch Ring-on-Ring tests to determine the pressure at which a sample breaks.
In the abraded test for instance, the sample is abraded with 90 grit silicon carbide sprayed on the sample with a 15 psi pressure. Alternatively, a controlled scratch can be introduced. This is then set into an instrument that has two concentric rings. The internal ring has the pressure increased and monitored until the glass fails.
Corning also has a fracture threshold test that uses a tiny micro-sized diamond point to introduce a micro defect into the sample at a certain pressure. The glass is then examined to see if the size of the defect enlarged or not. In the demo we saw using a 2 Kg weight, the soda lime glass saw an increase in the size of the defect while the GG4 sample did not.
This lab can also do 2-, 3- and 4-point bend testing of the glass. In a four-point test for example, the glass is fed through four rollers. The two at the ends are then moved up or down to create bending in the glass. This is used to determine failure points.
The lab even has what it calls a tumble test where a sample of items that might be in your pocket, like keys, change, pocket knives and pens, are tumbled with the glass sample to see what sort of damage is inflicted.
And no glass materials testing lab is complete without a ball drop test as well. Here, the idea is to drop a stainless steel ball of known weight from various heights multiple times until glass failure. They demonstrated how a 225g ball dropped on a GG4 sample from 1.5 meters survives the test (with the ball bouncing back up nearly half way).
The third and final lab on our tour is called the Mechanic Lab. This is where Corning conducts its drop testing. This is the most innovative lab where Corning has developed new methods to test and evaluate materials to try to replicate how devices are dropped in real life. In fact, these tests should be brought to the various standards bodies like ASTM and IEC, which Corning intends to do at some point. But for now, this is a proprietary process that the company has developed that offers a competitive advantage which Corning would like to maintain.
In the lab, the researchers use both real cell phones and what they call pucks. These are blocks of material with a glass mounted to them that look and feel like standard smartphones. These pucks can be mounted in the drop machines and dropped at various angles at various heights. Very high speed cameras are mounted at the bottom to capture the impact. Some videos of this impact were quite interesting showing flexing of the puck on impact http://tinyurl.com/kmh5btu.
Different surfaces can be used at the bottom of the drop including granite, stainless steel, concrete and asphalt. However, the Corning researchers discovered that repeated drop tests on rough surfaces like asphalt or concrete actually changes the surface of the materials. That means testing can tend to get better as the surface becomes smoother.
To eliminate this experimental bias, they have now standardized on using 180 grit sand paper. Their results confirmed this correlates well with asphalt and concrete, but is more controlled.
In a demo they gave in the lab, a puck using soda lime glass was dropped at an 8 degree angle. The soda lime sample broke after three drops from a meter, while the GG4 sample was dropped 3 times with no failure (in fact it had already been dropped 12 times before that). On average, Corning found that the soda lime glass breaks after 1.4 drops and the GG4 sample can survive 20 drops without breaking (they stop testing after 20 drops).
Analysis
Corning defines damage resistance as “the ability of the material to limit strength reduction due to flaws”. The strength was measured using the abraded and scratch ring-on-ring test described above.
Gorilla Glass 2 was introduced to improve the damage resistance of the original material. GG2 offered a 25% increase in damage resistance or the ability to use a glass that was 20% thinner but with the damage resistance of the original Gorilla Glass.
Gorilla Glass 3 was developed to improve the scratch resistance of GG2. As shown in the slide below, Corning was successful in this effort.
GG4 includes all the improvements of the previous generations, plus more. According to Dr. Jaymin Amin, a Division VP of Technology for Gorilla Glass, the goal of development was to dramatically reduce field failure rates, which required the development of all of the new testing methods.
GG4 now offers improved damage resistance over GG3 (2X improvement) and outperforms all glasses in the sharp contact damage. This latter category includes static and dynamic damage tests as well as drop tests.
The 1 meter drop test results are significant as this is a very common real life error (dropping while placing or pulling your phone from your pocket or pocket book). The researchers found that they improved the drop height to failure by 2X over GG3 and reduced the probability of failure from 85% with GG3 to 15% with GG4. That’s huge.
GG4 also allows handset or mobile device makers to not only improve performance with GG4, but to also use thinner glass at the same time.
These results are impressive and a key reason why GG4 should see widespread adoption going forward. – Chris Chinnock
