5.1 For advanced ceramics, Knoop indenters are used to create indentations. The surface projection of the long diagonal is measured with optical microscopes.
5.2 The Knoop indentation hardness is one of many properties that is used to characterize advanced ceramics. Attempts have been made to relate Knoop indentation hardness to other hardness scales, but no generally accepted methods are available. Such conversions are limited in scope and should be used with caution, except for special cases where a reliable basis for the conversion has been obtained by comparison tests.
5.3 For advanced ceramics, the Knoop indentation is often preferred to the Vickers indentation since the Knoop long diagonal length is 2.8 times longer than the Vickers diagonal for the same force, and cracking is much less of a problem (1).5 On the other hand, the long slender tip of the Knoop indentation is more difficult to precisely discern, especially in materials with low contrast. The indentation forces chosen in this test method are designed to produce indentations as large as may be possible with conventional microhardness equipment, yet not so large as to cause cracking.
5.4 The Knoop indentation is shallower than Vickers indentations made at the same force. Knoop indents may be useful in evaluating coating hardnesses.
5.5 Knoop hardness is calculated from the ratio of the applied force divided by the projected indentation area on the specimen surface. It is assumed that the elastic springback of the narrow diagonal is negligible. (Vickers indenters are also used to measure hardness, but Vickers hardness is calculated from the ratio of applied force to the area of contact of the four faces of the undeformed indenter.)
5.6 A full hardness characterization includes measurements over a broad range of indentation forces. Knoop hardness of ceramics usually decreases with increasing indentation size or indentation force such as that shown in Fig. 1.6 The trend is known as the indentation size effect (ISE). Hardness approaches a plateau constant hardness at sufficiently large indentation size or forces (loads). The test forces that are needed to achieve a constant hardness vary with the ceramic. The test force specified in this standard is intended to be sufficiently large that hardness is either close to or on the plateau, but not so large as to introduce excessive cracking. A comprehensive characterization of the ISE is recommended but is beyond the scope of this test method which measures hardness at a single, designated force.
FIG. 1 A Typical Indentation Size Effect (ISE) Curve for a Ceramic (The data shown are for NIST SRM 2830 silicon nitride)
1939年由前美国国家标准局的FrederickKnoop等人研制出的一种低试验力硬度试验,又称显微努氏硬度试验。试验力范围为:98.07X10-3N (0.01 kgf) ~ 9.807 N (1 kgf)。努氏压头使用菱形棱锥体金刚石压头,两个长棱之间的夹角为172˚30’,两个短棱之间的夹角为130˚,长短对角线长之比为7.114,短对角线长与压痕深度比为4.00。...
配备努氏压头后能测定玻璃、陶瓷、玛瑙、人造宝石等较脆而又硬材料的努氏硬度。...
显微维氏硬度测试验因其试验力比较小,更能进行材料金相组织及脆性材料的硬度测量。 关于产品可测维氏硬度的硬度计有VH1150维氏硬度计、VH1102/VH1202显微硬度计、VH3100/VH3300自动化维氏/努氏硬度计。详细内容,后文将结合努氏硬度部分介绍。 努氏硬度 美国的Knoop发明了努氏硬度,并以其名字命名。努氏硬度在我国列为小负荷硬度测试。...
配备努氏压头后能测定玻璃、陶瓷、玛瑙、人造宝石等较脆而又硬材料的努氏硬度。...
Copyright ©2007-2022 ANTPEDIA, All Rights Reserved
京ICP备07018254号 京公网安备1101085018 电信与信息服务业务经营许可证:京ICP证110310号