Page 315 - Handbook of Materials Failure Analysis

312 CHAPTER 13 A concise filtergram wear particle atlas

1 INTRODUCTION
Wear failure is a common failure in rotary machines and wear particle analysis is an
important technique in the wear diagnosis. Wear particle analysis is based on the fact
that any wear would generate wear particles (debris). As different wear mechanisms
generate different types of wear particles, the wear particles contain critical informa-
tion regarding the machine health. By analyzing the wear particles, the machinery
wear conditions can be assessed. In fact, wear particle analysis is a mature technique
with a history of over four decades. Nowadays, wear particle analysis has become a
necessary technique in advanced condition monitoring programs.
Wear particle analysis counts on three parameters, i.e., the particle types, sizes,
and quantity. Among the three parameters, particle-type identification is crucial.
Wear particle identification relies on the particle morphology and surface features.
The surface features include colors, texture, and other surface attributes. In terms of
wear particle analysis, the particle morphology and the surface features should be
taken account.
Broadly speaking, some other methods have also been categorized as wear par-
ticle analysis, such as elemental analysis, automatic particle count, and PQ (particle
quantify) index. Strictly speaking, as those methods are irrelevant to wear particle
morphology, the name of wear analysis, rather than wear particle analysis, is more
appropriate to those methods.
To successfully assess the particle type, size, and quantity, ferrography, filter-
gram (patch test), and computer image analysis constitute the three major techniques
of wear particle analysis in modern-day condition monitoring [1].

2 THE WEAR PARTICLE ANALYSIS TECHNIQUES
Ferrography: Invented and developed in 1970s, ferrography ushered in a new era for
tribology. Since its invention, wear particles have been extensively studied under the
microscope. In ferrography, a magnetic field is applied to separate wear particles
from the oil. Since ferrography was successfully applied for first time in wear diag-
nosis in the US military in 1970s [2], wear particle analysis has been widely applied
in various industries from 1980s to 1990s.
There are two main types of ferrography instruments, the analytical ferrography
and the DR (direct reading) ferrography. DR ferrography is now obsolete and has
almost completely replaced by PQ in industry. In the present day, ferrography gen-
erally refers to analytical ferrography.
Analytical ferrography applies a magnetic field to precipitate particles onto a
glass slide called a ferrogram. Under the influence of magnetic forces, the magne-
tized particles are deposited in strings and sorted by their sizes at different positions.
The particles on the ferrogram are examined under an optical microscope with
bichromatic lights. Green color is applied for the transmission light, while red or
white color is for the reflection light. Due to the bichromatic lights simplifying