You may have heard of the zirconia material being used in industrial components or dental crowns because of its toughness. But have you ever wondered why this ceramic is so resilient? The secret lies in a process called transformation toughening. In this article, you’ll learn how it works.
How Does the Zirconia Transformation Toughening Process Work?
Also known as Zirconium Dioxide, Zirconia is a volatile material that exists in three different crystal phases.
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The monoclinic phase (when it’s at room temperature)
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Tetragonal phase (when it’s at a higher temperature)
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Cubic phase (when it’s at a very high temperature)
It works like this - when zirconia (zro2) cools from high temperatures, it transforms from the tetragonal to the monoclinic phase. While this happens, there’s a volume expansion within the range of 3-4%, which can cause cracks in the material. Now, that’s undesirable, but under certain conditions, this transformation can be controlled to work in your favor.
Here’s the trick - keep zro2 ceramic in its tetragonal phase by adding stabilizers like Yttria (Y₂O₃), Magnesia (MgO), etc. These elements control the grain size to prevent unwanted phase transformations. As a result, zirconia hardness will be sustainable regardless of the change in temperature.
When this happens, you can trigger its transformation to the monoclinic phase only when you apply stress. So, when a crack starts to form, the stress surrounding the tip will trigger its conversion to the monoclinic phase.
The transformation comes with a volume expansion that seals the crack, making it harder for it to spread. In a nutshell, a transformation toughened zirconia has a defense mechanism that prevents it from fracturing or cracking. While other materials get weaker when they crack, Zirconium oxide ceramic gets stronger.
The Real-World Applications of Transformation Toughening Zirconia
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Dental implants, bridges, and crowns: Zirconia is used for your dental prosthetics because it can withstand the stress of chewing without chips or cracks. As a plus, it’s biocompatible, so there will be no adverse reactions in your body.
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Cutting tools: Metalworkers and machinists prefer zirconia when manufacturing knives and blades because they’re incredibly durable. Also, they perform better when you use them in high-speed cutting conditions that may cause other materials to crack.
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Thermal barrier coatings: Zirconium oxide thermal conductivity is pretty low, which this makes it the perfect heat insulator. For this reason, aerospace engineers use it as a thermal barrier coating for jet engines and gas turbines. It will protect metal components from extreme heat and thermal cycling and improve their lifespan.
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Orthopedic implants: Zirconia is used for your hip joints and knee replacements because of its stress resistance and wear in bodily movements.
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Automotive components: Some of your engine components, fuel injector parts, and valves are made with zirconia material for a high wear resistance. Zirconia crystal structure is perfect for high-performance engines because it can withstand high temperatures and mechanical stress.
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Surgical instruments: This material is used to manufacture tweezers, scissors, and scalpels to keep them sharp, non-corrosive, and wear-resistant. As a plus, they have a long lifespan and are resilient to fractures even after multiple sterilization cycles.
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Industrial seals and bearings: Zirconium dioxide is used to make bearings and seals that can withstand heavy loads, friction, and harsh environments.
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Luxury watches: Zirconia is used to make many of the luxury watch cases you know. They prefer this material because it makes the cases lightweight, scratch-resistant, visually appealing, and durable.
Most times, manufacturers have to settle for the superior material in zirconia vs ceramic sanding belts. However, zirconia comes out on top if you’re talking about high-stress applications.
The Challenges of Zirconia Transformation Toughening and Their Solutions
Low-Temperature Degradation (Aging)
Aging is the primary challenge associated with the transformation toughening process. You see, when zirconia is exposed to moisture and low temperatures, an unwanted phase transformation will happen. As a result, it will be weaker, and you’ll see micro-cracks, leading to rough surfaces.
The Solution: Use high-purity materials with advanced stabilizers like ceria (CeO₂) instead of the traditional yttria (Y₂O₃). When you do this, it will improve the zirconia’s resistance to aging.
Zirconia Becomes Brittle if the Grain Sizes are Too Large
Transformation-toughened zirconia can lose its toughness if the grain sizes expand excessively in the tetragonal phase. The larger the grain, the more prone the material will be to spontaneous transformations without external stress. As a result, you’ll notice that the material will become more susceptible to cracking.
You may be wondering why zirconia ceramic, which is supposed to be tough, can have large grain sizes. Well, it’s possible that the material didn’t go through proper sintering processes during manufacturing. So, the particles won’t be evenly heated below the zro2 melting point, resulting in high grain sizes. You can’t use such zirconia for high-stress applications such as engine components or cutting tools.
The Solution: Use nano-grained zirconia. Alternatively, you can carefully control sintering temperatures and times to achieve the optimal grain size.
High Manufacturing Costs
The production of transformation toughened zirconia involves complex processes. You have to dope with stabilizers and be precise in sintering the material, which is much harder than it sounds. You also need to maintain strict control over its grain size, phase stability, and purity. Because of that, zirconia is more expensive than other ceramic materials.
The Solution: Researchers are working on 3D printing and improving sintering efficiency to lower manufacturing costs.
Limited Thermal Shock Resistance
Although zirconia is tough, it’s still prone to thermal shock if you expose it to quick temperature changes. When thermal shock happens, you’ll see microcracks on the surface, especially if it’s a thin component.
The Solution: Use graded thermal coatings or hybrid ceramics to improve the thermal shock resistance.
What are the Recent Advances in Transformation Toughening?
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Nano-grained zirconia - to enhance the toughening effect
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Advanced dopants - to improve resistance to aging and toughness
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Hybrid ceramics - combining zirconia with other materials for better performance in certain applications.
Final Words
The transformation toughening process is what distinguishes zirconia from other ceramics because it doesn’t just make it resilient but self-reinforcing. So when next you hear of zirconia, you’ll know why it’s tough enough to stop cracks in their tracks.