Photo: KoolShooters

Why is bond strength important?

Increased bond strengths are the major element that has made composites successful and predictable. These materials are also more polishable and are available in a wide range of shades. This is important considering patient desires to make teeth look as white as possible.

sciencedirect.com - Bond Strength - an overview | ScienceDirect Topics

What does God require from man?

And what does the LORD require of you? To act justly and to love mercy and to walk humbly with your God” (Micah 6:8). “Let us hear the conclusion...

Last updated Jul 2, 2018

Why do guys disappear then come back?

When a guy disappears and then comes back, it most likely means he's taking a break because it can be taxing to be in a relationship, and sometimes...

Last updated Nov 25, 2020

Promotion

Metal-Ceramic Bonding

The bond strength between porcelain and metal is an important requirement for good long-term performance of metal-ceramic restorations. In general, the bond is a result of chemisorption by diffusion between the surface oxide layer on the alloy and the porcelain. For metal alloys that do not oxidize easily, this oxide layer is formed during a special firing cycle prior to opaque porcelain application. For metal alloys that do oxidize easily, the oxide layer is formed during wetting of the alloy by the porcelain and subsequent firing cycle. The most common mechanical failure for metal-ceramic restorations is debonding of the porcelain from the metal. Many factors control metal-ceramic adhesion: the formation of strong chemical bond, mechanical interlocking between the two materials, and thermal residual stresses. In addition, as noted earlier, the porcelain must wet and fuse to the surface to form a uniform interface with no voids. These factors are also important for ceramic coatings on metallic implants. From a practical standpoint, the surface roughness at the metal-ceramic interface has a large effect on the quality of the metal-ceramic bond. Airborne particle abrasion is routinely used on metal frameworks for metal-ceramic restorations to produce a clean surface with controlled roughness. During the firing cycle, the porcelain softens, its viscosity decreases, and the porcelain first wets the metal surface before interlocking between porcelain and metal is created. The increased area of the rough metal surface also permits the formation of a greater density of chemical bonds. The contact angle between the porcelain and metal is a measure of the wetting and, to some extent, the quality of the bond that forms. Low contact angles indicate good wetting. The contact angle of porcelain on a gold (Au) alloy is about 60 degrees. A scanning electron micrograph of the oxidized surface of a gold (Au)-platinum (Pt)-palladium (Pd) 98% noble alloy is shown in Figure 10-10. However, rough surfaces can reduce adhesion if the porcelain does not wet the surface and voids are present at the interface. The formation of an oxide layer at the surface of the metal has been shown to be the key to an adequate metal-ceramic bond. Noble metal alloys, which are resistant to oxidizing, usually have other more easily oxidized elements added, such as indium (In) and tin (Sn), to form an oxide layer and improve the bond. The oxide layer is formed during a special firing cycle prior to porcelain application. Some noble alloys containing silver have been shown to lead to porcelain discoloration or greening, explained by ionic diffusion of silver in the porcelain. Base-metal alloys contain elements, such as nickel (Ni) and chromium (Cr), that oxidize easily, and care must be taken to avoid the formation of too thick an oxide layer. Manufacturers specify firing conditions for the formation of an optimal oxide layer and often indicate the color of the oxide. Oxides rich in nickel (NiO) tend to be dark gray, whereas those rich in chromium (Cr 2 O 3 ) are greenish. If firing recommendations are not followed, these oxides may dissolve in the porcelain during firing, leading to discoloration visible in areas where the porcelain is thinnest, for example near the gingival margin of the restoration. Some alloys form oxide layers rich in Cr 2 O 3 , which do not bond or adhere well to the alloy. These alloys typically require the application of a bonding agent to the alloy surface to modify the type of oxide formed. In some cases, manufacturers recommend an oxidation firing under reduced pressure to limit the thickness of the oxide layer. An oxidation firing in air may lead to a thicker oxide layer.

Why do men stay away from you?

One of the reasons why men pull away is because they don't feel an intense emotional connection. A guy might like you for a while and start to keep...

Last updated May 21, 2022

Are people with multiple partners happier?

The benefits of one partner are slightly greater for men than for women: according to the baseline results, 71% of men with one partner are very...

Last updated Jun 27, 2020

Promotion

High thermal residual stresses between the metal and porcelain can lead to failure. If the metal and ceramic have largely different thermal expansion coefficients, the two materials will contract at different rates during cooling and large thermal residual stresses will form along the metal-ceramic interface. If these stresses are very high (whether tensile or compressive), the porcelain will crack and/or delaminate from the metal. Even if these stresses do not cause immediate failure, they can still weaken the bond, and lead to delayed failure. To avoid these problems, porcelains and alloys are formulated to have adequately matched thermal expansion coefficients. Most porcelains have coefficients of thermal expansion between 13.0 and 14.0 × 10−6/° C, and metals between 13.5 and 14.5 × 10−6/° C. The difference of 0.5 × 10−6/° C in thermal contraction between metal and porcelain causes the metal to contract slightly more than does the ceramic during cooling. This condition places the porcelain under slight residual compression, which makes it less sensitive to the tensile stresses induced by mechanical loading. A metal-ceramic bond may fail in any of three possible locations (Figure 11-10). Knowing the location of failure provides considerable information on the quality of the bond. The highest bond strength leads to failure within the porcelain when tested (see Figure 11-10, C); this is observed with some alloys that were properly prepared with excellent wetting by the porcelain and is also called a cohesive failure. Testing these high-strength specimens using the push-through shear test shows that the bond strength is approximately the same as the shear strength of the porcelain. Another possible cohesive failure is within the oxide layer (see Figure 11-10, B). Failures occurring at the interface between metal and oxide layer (see Figure 11-10, A) are called adhesive failures and are commonly observed with metal alloys that are resistant to forming surface oxides, such as pure gold or platinum, and exhibit poor bonding. Base-metal alloys commonly exhibit failures within the oxide layer if an excessively thick oxide layer is present.

What to avoid online dating?

When Connecting Online Use different photos for your dating profile. ... Avoid connecting with suspicious profiles. ... Check out your potential...

Last updated Oct 20, 2018

What 10 things men want from a woman?

Sex. Admit it — you saw this one coming. ... Honest reactions. Don't fake it with us. ... Compliments. Men want to be complimented just as much as...

Last updated Aug 26, 2018

Promotion

sciencedirect.com - Bond Strength - an overview | ScienceDirect Topics

What does a good kisser feel like?

Good kissers are soft but passionate. They practice good hygiene, know how to move their lips and tongue to make the kiss feel passionate. They...

Last updated Feb 10, 2019

When you pray for those who hurt you?

“Love your enemies and pray for those who persecute you, that you may be children of your Father in heaven,” are direct instructions from Jesus in...

Last updated Jun 28, 2018

Promotion

What is the most attractive thing in a girl?

8 Highly Attractive Things In Women (That Have Nothing to Do With Appearance) Kindness. ... Positivity and sense of humor. ... Passion. ... Getting...

Last updated Jan 6, 2019

Promotion

What are the five problems of marriage?

7 Of the Most Common Marital Problems Money. Of all the common marital problems, it's no surprise that money is at the root more often than not....

Last updated Jul 5, 2022