Hardness testing systems

Many different testing methods have been described for hardness testing. Most are based upon the ability of the surface of a material to resist penetration by a point under a specified load.

One of the oldest tests employed is the Brinell test. This method depends on the resistance offered to the penetration of a small steel ball, 1.6 mm in diameter, when subjected to a weight of 123 N (27.71bs). The ball penetrator is allowed to remain in contact with the sample tested for a fixed time o f 30 seconds (Craig 1980). The load is divided by the area of the surface of the indentation to give the Brinell Hardness Number. Thus the smaller the indentation, the larger is the hardness number, and the harder the material. This test has been used extensively for determining the hardness of metals and metallic materials in dentistry. It is not suitable for determining the hardness of brittle materials or those that exhibit significant elastic strain. Thus the Brinell test is not suitable for measuring the hardness of tooth structure and cement, which are brittle materials, or the dental plastics which exhibit elastic recovery. As it is not possible to identify indenting marks clearly in a ‘plastic' material.

The Rockwell hardness test, similar to the Brinell test with a stainless steel ball, has been used by some workers for microfilled visible light cured composite resins (Raptis et al. 1979), where the depth of the indentation is measured directly by a dial gauge on the instrument. As with the Brinell test it is not regarded as suitable for brittle materials.

The Knoop hardness test has been used more widely in testing the hardness of polymeric materials and employs a diamond indenting tool that is cut in a rhombic geometrical outline. The length of the largest diagonal is measured. The theory on which the test is based is that when the indentation is made, a cutting action occurs along the major axis o f the impression, and spreading takes place along the minor axis. The stresses are therefore distributed in such a manner that primarily only the dimensions of the minor axis are subject to change by relaxation. Thus the hardness value is virtually independant of the ductility o f the material tested. This potentially means that the hardness of tooth enamel can be compared with that of gold, porcelain, resin, and other tooth restorative materials. This testing system has been used by a number of workers on both microfilled and hybrid resin materials (Li et al.

The alternative test suitable for assessing the hardness o f rigid and semi-rigid plastics and the state of cure of synthetic resins (Young et al. 1978) is a micro hardness test using the Wallace Micro-Indentation Hardness Test (H.W. Wallace & Co., Croydon, U.K.). Both this and the Knoop hardness test are classified as microhardness tests when compared with the Brinell and Rockwell tests. The two techniques employ loads of less than 9.BN, and the resulting indentations are small and are limited to a depth of less than 19 micrometers (Young et al. 1978). Hence they are capable of measuring very thin objects and giving hardness values of small regions. The Wallace hardness tester has been used over perhaps the widest range of polymeric materials, including fissure sealants (Young et al. 1978), experimental composites with varying filler fractions (Braem et al. 1989), visible light cured special tray resin composites, (Devlin et al. 1995), microfilled composites (Jorgensen 1980), and in correlation between depth and cure, degree of conversion and hardness in microfilled light activated resins (Asmussen 1982B; Hansen and Asmussen 1993). The composite disc samples usually of 1-4 mm in thickness and 10-20 mm in diameter are loaded with a Vickers diamond pyramid with a primary load of 1 gram for 15 seconds, followed by the main load of 100-300 grams for 60 seconds. The primary load fixes the position and helps to ensure an even contact. The indentation depths in micrometers are read at the end of the 60 seconds main loading.

Many o f the studies on surface hardness specifically relate to the relationship between the value of hardness and smooth surface resins (Asmussen 1982B; Jorgensen 1980). Most recently produced composite resins of the hybrid or microfilled type contain particles of 1 micrometer or less in size, and testing of the surface hardness is done with microhardness testing apparatus, as the values require the indenter to be measuring the surface hardness o f resin and not filler particles. Composites with relatively large inorganic filler particles are not suitable for this apparatus; as it has been found meaningless to apply a microhardness test on these mechanically highly heterogeneous materials (Jorgensen 1980). However, it has been used on type 111 cast gold alloys and several amalgam formulations. The primary problem with any indentation method is ensuring that the indenter does in fact indent the surface of the resin and not a filler particle, since the indenting test is for determining how hard the resin component is after polymerisation.

The wide variety of hardness values and methodologies in the literature make inter study comparisons rather unfruitful (Yearn 1985). However, as a measure for clinical abrasion Jorgensen (1980) has suggested that the Wallace hardness test is considered suitable for comparison of hardness in smooth-surface resins, amalgam and gold alloys.