Manual of Laboratory Testing Methods for Dental Restorative Materials. Paromita Mazumdar

situated within a bigger matrix and testing foil‐like or thin materials.

      The usual method to achieve hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape with a specific force applied for a specific time.

      Vickers and Knoop both involve the use of diamond pyramid indenters. In the case of Vickers hardness, the diamond pyramid has a square base, whilst for Knoop hardness, one axis of the diamond pyramid is much larger than the other.

      The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136° between opposite faces subjected to a load of 1–100 Kgf. The load is normally applied for 10–15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average is calculated. The area of the sloping surface of the indentation is calculated. It is suitable to be applied to determine the hardness of small areas and for very hard materials.

      Knoop hardness is more sensitive to surface characteristics of the material. The Knoop indenter is a diamond ground to pyramidal form that produces a diamond‐shaped indentation having approximate ratio between long and short diagonals of 7 : 1. The depth of indentation is about 1/30 of its length. When measuring the Knoop hardness, only the longest diagonal of the indentation is measured, and this is used in the following formula with the load used to calculate Knoop Hardness Number (KHN). Knoop hardness test is applied to evaluate enamel and dentine structures. One of the major difficulties is the requirement of a high polished flat surface that is more time‐consuming and more care taking compared to other tests.

      Comparing the indentations made with Knoop and Vickers Diamond Pyramid indenters for a given load and test material, there are some technical differences as follows:

       Vickers indenter penetrates about twice as deep as Knoop indenter.

       Vickers indentation diagonal is about 1/3 of the length of Knoop major diagonal.Figure 1.12 Vickers hardness testing.Figure 1.13 Knoop hardness testing.Figure 1.14 Vickers hardness tester.

       Vickers test is less sensitive to surface conditions than Knoop test.

       Vickers test is more sensitive to measurement errors than Knoop test.

       Vickers test is best for small rounded areas, whereas Knoop test is best for small elongated areas (as shown in Figures 1.12–1.14).

The Brinell hardness test method consists of indenting the material with a 10 mm diameter hardened steel or carbide ball subjected to a load. It is the oldest method to measure surface hardness and is applicable to test metals and alloys (as shown in Figure 1.15).

Measurements are normally made using a microscope since the indentations are often too small to be seen with the naked eye. The hardness is a function of the diameter of the circle for Brinell hardness and the distance across the diagonal axes for Vickers and Knoop hardness. Allowance is naturally made for the magnitude of the applied loads. In the case of Rockwell hardness, a direct measurement of the depth of penetration of a conical diamond indenter is made. The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. This method is useful to evaluate surface hardness of plastic materials used in dentistry [4] (as shown in Figure 1.16).

Figure 1.15 Brinell hardness testing.

Figure 1.16 Rockwell hardness testing.

      Source: Vieira [4]. Licensed under CC BY 4.0.

       Good to Know

      All materials require different hardness testing. The specificity of a hardness tester is dependent on the following factors:

      1 Material of the indenter

      2 Shape and size of the indenter and the sample to be tested.

      3 Loading parameter (amount of force it can apply)

      Hardness tests are extremely used and have important applicability on Dentistry. Hardness test can evaluate the degree of mineralization of a dental substrate for example. A specific force applied for a specific time and distance provides important data in studies assessing the ability of enamel and dentin remineralization after different treatments as happens in unbalanced situations of des‐remineralization. Another important use of this test is to evaluate the degree of polymerization of resin composite and resin cements.

Figure 1.17 Stress–strain curve for assessment of Young's modulus.

1.7 Elastic Modulus

      The modulus of elasticity, or measure of a material's stiffness, is also important in relation to anticipated longevity of a restoration. An elastic material (one with a low elastic modulus) will deform when a load is placed on it but will return to its original shape once the load falls below the elastic limit of the material. As a general rule, restorative materials need to be very stiff (high elastic modulus), so that under load the elastic deformation will be very small. An exception to this is in the Class V situation. Micro‐filled composite materials have a lower modulus of elasticity than hybrid composite materials; this may be why micro‐filled materials show higher retention rates in Class V cavities, given that they deform more readily as the tooth deforms at the cervical area under occlusal loading.

Models involving the use of springs and dashpots can be used to explain the elastic and viscoelastic behaviour of materials. When a spring, which represents an elastic material, is fixed at one end and a load applied at the other, it becomes instantaneously extended. When the load is removed, it immediately recovers its original length. This behaviour is analogous to that of a perfectly elastic material. The two things that characterize the material are firstly the perfect recovery after removal of the force and secondly the lack of any time dependency of either the deformation under load or the recovery after removal of the applied force. The extent of deformation under load is characterized by the modulus of elasticity of the material (analogous to the spring constant of the spring) (as shown in Figure 1.17).

1.8 Fracture Toughness

Fracture toughness determines the resistance of a material to the propagation of a crack. This test has been considered to be efficient given that other parameters can be derived from it. It should be kept in mind, however, that fracture toughness measures the failure of a material after one continuous period of loading, whereas fatigue strength experiments measure crack propagation after repeated applications of a small cyclic load. A UTM