The Metal/Ceramic Bond

Transcription

1 The Metal/Ceramic Bond

2 Introduction Although ceramic restorations have been used in dentistry since 1903, it was not until the 1950 s that Porcelain-Fused-to-Metal restorations (PFM technique) came into use, sometimes called the metallo-ceramic restoration. This involves casting a metal framework, after waxing it up on a stone model of the patient s dentition. The ceramic powder is then applied to the metal surface and fired. The technique is more properly regarded as a type of enamelling work. The advantage over full ceramic restorations is that the metal framework adds its toughness and flexural resistance to support the ceramic.

3 Introduction The critical part of PFM work is the creation of a satisfactory bond between the metal and the ceramic layer fired onto it. Properly done, this bond can be as strong as the materials used themselves. The strength of the bond is developed by several basic mechanisms. The mechanisms are: The Mechanical Bond The Chemical Bond The Physical Bond

4 Introduction Following below is the explanation of the metal ceramic bond mechanisms and some other contributing factors such as: The Surface Interactions The importance of coefficient of expansion (Physical Bond) The structure of the bond layer The definition of Cohesive and Adhesive failure The critical contribution of the metal oxide layer (Chemical Bond)

5 The Mechanical Bond The preparation for metal/ceramic bonding involves abrading the surface of the cleaned metal casting. An abrasive stone is used, and the intention is not to make a fine polish, but to introduce a controlled amount of surface roughness. The abrasive produces a series of scratches in the surface, which have two benefits for the bonding process: 1. They increase the actual area of the metal surface which is available to be bonded. 2. They provide some undercut grooves into which the soft ceramic will flow into during firing. The ceramic will be locked in the keys after bonding and cooling down.

6 The Mechanical Bond

7 The Chemical Bond The mechanical bond mechanism increases the effective strength of a bond, but it does not make a bond. The chemical mechanism of the bond involves the oxide layer which has been carefully grown on the surface of the metal during a previous brief high temperature heat treatment. This oxide layer is formed as a chemical compound, so its oxide atoms are bonded to the metal atoms in the surface of the metal. During firing the oxide is gradually consumed by the ceramic layer.

8 The Chemical Bond Atoms from this layer move by diffusion into the structure of the silicate glass which is the ceramic and become ionically bonded into it as intermediates or modifiers. However, the remaining part of the oxide layer not consumed by the ceramic is still chemically bonded to the metal. The bond region thus becomes a gradual change in composition from metal to ceramic. There is no region which does not have chemical bonds holding it together.

9 The Surface Interactions The mechanical and chemical bonding mechanisms require that the metal and ceramic materials are brought into intimate contact before the bond will work. It is not an exaggeration to say that the metal and ceramic must be at an atomic level of contact. This means that their surfaces must not repel each other. The soft, flowing ceramic must wet the metal surface. If it does not, there will be areas where the surfaces are not in contact and bubbles or small voids appear. No bond can develop at a point where the metal and ceramic do not touch each other, and cracks may begin from the point.

10 The Surface Interactions To achieve this intimate degree of surface wetting, the surface tension of the ceramic and the metal oxide must be compatible, and there must be no impurities at all present. Burnt on carbonaceous material in particular will leave a small unwetted region around it, so any grease, oil, sweat, or similar impurities must be removed. It is important that the metal surface after oxidising must not be contaminated with dust, organic material, or any other impurity. Only laboratory cleanliness and sound technique can prevent this.

11 The Importance of The Coefficient of Expansion A satisfactory match of the coefficient of expansions (ie. the metal coefficient and ceramic coefficient) will avoid causing cracks forming in the ceramic as it cools. It will also avoid stresses being created in the ceramic which will reduce its resistance to cracking while in service. If the ceramic has a smaller coefficient of expansion than the metal, then when it is cooling after firing, the metal will contract more. This will put the ceramic layer under compression, which can be strong enough to break the bond region and flip the ceramic layer off.

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13 The Importance of The Coefficient of Expansion If the ceramic has a higher coefficient of expansion than the metal, it will try to contract more when cooling after firing, and will put itself under tension. Since the ceramic is weak in tension, this can quite easily cause large cracks in the layer, or a pattern of many smaller ones, which will at least let in stains if not cause complete failure.

14 The Importance of The Coefficient of Expansion The best match of metal and ceramic coefficients of expansion occurs where the ceramic has a very slightly smaller coefficient of expansion than the metal. This puts the ceramic layer under some compression as it cools. These smaller compressive stresses locked in during cooling will not be sufficient to damage the ceramic, but will increase its resistance to the tensile stresses produced by bending while in service. Strictly speaking, matching the coefficients of expansion this way does not make the bond stronger, but it does make the ceramic less likely to crack during fabrication and service.

15 The Structure of the Bond Layer The Bonding between the different materials can be represented using this simple block diagram. The question we have to ask is, if the metal/ceramic restoration fails, where did it fail?

16 Definition of Cohesive and Adhesive Failure Look at the diagram. If failure occurred at regions 1, 3, or 5, this would be called a cohesive failure, because it was a failure within the one material; a failure of the metal, ceramic, or oxide to hold together under the applied stress.

17 Definition of Cohesive and Adhesive Failure If failure occurred at the interface regions, 2 or 4, it would be called an adhesive failure, because of failure of the two materials across the interface to hold together.

18 The Critical Contribution of The Metal Oxide Layer Not all oxides forming on metals are suitable for this bonding technique. For example, the oxide which forms on chrome alloys at high temperatures is a thick, green, porous material. Its own strength is small, and it makes a weak link in the bonding layers. The oxide which forms on chrome alloys at lower temperatures, and even at room temperature is thinner and not porous. It is much stronger and forms a satisfactory bond.