Windshields are a crucial piece of safety equipment on aircraft, cars, buses and other vehicles. Aerodynamically designed, windshields typically consist of an inner cellulose layer covered by two layers of glass; one on the outside and one on the inside. Since shattered glass from windshields can cause serious injuries or fatalities when a collision occurs, it's important that the vehicle's windshield be strong. Referring to the Mohs scale is a good way to discern the relative toughness of a windshield.
What Is the Mohs Scale?
Friedrich Mohs, a German mineralogist, developed the Mohs scale in 1812 by comparing the hardness of ten common minerals. The scale ranges from 1 to 10, with one being the least hard and ten being the hardest. Talc, which is used to make talcum powder, a commonly used substance in beauty products, is rated at 1 on the hardness scale. Calcite, a mineral found in limestone, scores a 3. Quartz, the most prevalent mineral in the crust of the earth, is a 7. Diamond scores a 10. Deciding the hardness of substance or everyday object involves making comparisons with these minerals. For instance, the human fingernail scores about a 2 to 2.5. A hardened steel file is roughly a 6.5.
Nowadays, most windshields manufactured for cars in Australia don't go below 5.5 on the Mohs scale. It's advised to check before buying, though, as any windshield less than 5.5 isn't hard enough to keep from breaking and can be dangerous if a wreck happens.
Why It's Important to Know How Hard a Windshield Is on the Mohs Scale
As mentioned earlier, having a solidly constructed windshield is vital to safety in a vehicle. The windshield must not crack or shatter easily from road debris, hail or other flying objects. It's important to note that, no matter how strong the windshield is advertised to be, it will undoubtedly break in heavy impact collisions. But it must break safely, so that when it breaks, large, sharp pieces don't come into contact with the driver and passengers. This is why most windshields in Australia are now designed with three layers.
Additionally, when cleaning a windshield, it's important to not use steel wool or other scrubbing materials that rates higher than windshield glass on the Mohs scale. This will scratch and damage the windshield.
Ratings for Different Types of Windshields on the Mohs Scale
There are two main types of windshields: tempered glass windshields and laminated safety glass windshields (more common). Bulletproof windshields are a third option, but are relatively uncommon. The ratings of these vary on the Mohs scale.
Laminated Safety Glass Windshields
In a laminated windshield, a layer of clear plastic (typically cellulose-based) is blanketed by two standard glass planes. This type of windshield is most common and scores between a 6 and 6.5 on the Mohs scale, making it harder than regular glass (5.5), but not as hard as emerald (7.5). Though this kind scratches somewhat easily, they're designed well. When the windshield breaks in a collision, the interior plastic layer holds the glass shards, protecting the driver and passenger from getting cut.
Tempered Glass Windshields
Heat-treated to harden the glass surface (on the outside, not inside) and enhance scratch resistance, tempered glass windshields are harder than laminated varieties, rating a 7 on the Mohs scale. The tempering also has another effect; when the glass breaks, it's designed to shatter into tiny chunks instead of big jagged pieces. This decreases the likelihood of serious injuries.
Bullet-Proof Glass Windshields
Though not a common option, bullet-proof glass windshields are the hardest on the Mohs scale (around 8, though some manufacturers may claim higher). A combination of laminated glass and tempered glass, bullet-proof glass windshields also contain polycarbonates and thermoplastic for added hardness and protection.