It has been estimated that in the U S . economy the damage done by wear alone represents about 6% of the gross national product (I).Thus,the development of effeetive techniques for the reliable analysis of wear metal debris is of substantial interest to analytical chemists. Traditionally, spectrometric oil analysis has been performed in a manner that is generally unsuitable for the reliable detection of severe wear. The analvst simulv takes a sample of lube oil from the'equipment to a spectrometer and runs it. 1086A
The analytical data obtained in this manner may not reflect the extent of internal wear within the oil-wetted system being monitored, particularly when a component is undergoing ex. tensive wear that will soon lead to catastrophic failure. These errors in analyses are especially critical when the equipment being monitored is an aircraft, particularly a single-engine air. craft whose engine is about to fail inflieht.catagtroohicallv.with the wt e i t i d loss of 60th ai&& and c;ew. Undetected failures both of aircraft
ANALYTICAL CHEMISTRY, VOL. 56. NO. 9. AWJST 1984
and ground equipment can also result in substantial cost outlays and unplanned downtime once the equipment fails. Most analysts are not aware of the multitude of factors that affect the quality of data obtained in wear metal determinations. In this article we will discuss several of these factors and summarize the results of recent research that bear directly on the quality of the analytical data obtained, in the hope that it will be of value in the numerous applications of wear metal determinations. 0005270018UA35 1-1086$01.50/0 0 1984 American Chemical Society
Kent J. Eisenlraut Rebecca W. Newman Air Fwce Wright Aeronautical Laboratories United States Air Force Wright-Patterson Air Force h e , Ohio 45433 Costandy S. Saba Robert E. Kauffman Wendell E. Rhine University of Dayton Research Institute Dayton, Ohio 45469
Spectrometric oil analysis was first applied by the railroads in the early 19408 in an effort to detect wear in diesel locomotive engines. The technique was later adopted by the U.S. military. In the early 1960s spectrometric oil analysis was applied successfully to the detection of wear in turbojet aircraft engines by the US. Air Force. Considering the tremendous cost savings to be achieved in r e duced maintenance and repair activities, as well as improved operational reliability of weapon systems, the Ar i Force has maintained a continued interest in research to improve the capability to detect and identify the extent of aircraft engine internal wear. Currently, the U.S. military services use rotating-disk electrode atomic emission spectrometry (RDE-AES) and, to a much lesaer extent, flame atomic absorption spectrophotometry (AAS)as primary analytical techniques for wear metal determinations. Both of these techniques as currently applied have limitations in providing accurate data for wear metal content of engine oil samples (2-8).
Interferences Emission Spectrometry may suffer from interelement and matrix interferences. Interelement interferences cause enhancement or suppression of the analytical signal due to the presence of concomitant elements in the sample or standard. The normal procedure is to calibrate the spectrometer with a multielement standard and then analyze samples that may contain one or more of these elements. Thii procedure can lead to errors, as depicted in Table I. Matrix interferencea may be caused by differences in chemical composition between the standard and the sample to be analyzed, as shown in Figure 1.The data
Figure 1. Concentration of Fe determined by RDE-AES in differentlubricating oil shown in thii figure represent the RDF-AES readout in ppm Fe for the same 20-element Conhstan standard in different lubricating oil matrices. When the instrument is standardized with the 20-element standsld in Conostan 245 base oil, 100 ppm of Fe reads 100 ppm. When thesame concentration of iron is determined using the same 20-element standard in different oil matrices, the concentration of Fe indicated on the spectrometer readout increases. For example, with MIL-L-23699 lubricating oil as the
20-element standard matrix, the 100ppm Fe standard reads approximately 200 ppm Fe. ~~
Wear Detection A further limitation of AES and AAS for wear metal determinations is that both techniques are affected by the particle size of the wear metal. In actual oil samples the wear metal can be present as dissolved species or range to millimeter-sized particles. The detection and identification of metallic particles in the lubricant are
ANALYTICAL CHEMISTRY. VOL. 56, NO. 9. AUGUST 1984
1087A
lame I. tnect or r;oncomitani tiements on nut-Ats E-l
Ag AI
Ba Be cd
cr cu Fe
Ms
W a r d e q l h (A)
Cone. (ppn)
3281 3028 4554 2651 2265 4254 3274 2599 2803
137 97 166 95 210 85 185 134 106
E*nwnl
Mn
Mo
Na Ni Pb
Si Sn Ti
v
Wave-
(A)
2576 2816 5896 3415 2833 2516 3175 3349 4379
-
Cons. (ppn)
131 170 191 142 133 107 126 148 94
a RE-AES readout of Singleelementstandard of 100 ppn in MR-L-7808H s y n m n i ~ nil Instnment calibrated WW? 20-elsment standard in MILL-7808H syndretic oil at 100 ppm.
of critical importance because their presence is indicative of engine oilwetted component wear. Another critical factor is that the presence of metdlic wear particles can in turn further degrade component surfaces. The
inability to detect large wear particles bas led to engine component failure without prior indication by spectrometric methods. Several cases have been documented where large metallic wear particles produced by severe-
wear mechanisms have not been quantitatively determined by the spectrometric techniques employed (6,8-11). Both AES and flame AAS techniques are limited in their ability to quantitatively determine metal particles in oil samples when the particle size is larger than -1-8 pm, depending on the specific metal. The limitations for iron metal powder are shown in Figure 2. In this figure the concentrations of iron detected directly with various spectrometric techniques are plotted against the particle size of iron present in the sample. These data should be compared with the actual concentration present as determined by acid dissolution of the Fe particles using the particle-sue-independent method (PSIM) (6).The only analysis technique capable of directly determining iron particles of submicrometer to -2%30-pm size (unfiltered) is graphite furnace atomic absorption (HGA). Flame AAS using nitrous oxideacetylene and inductively coupled plasma atomic emission spectrometry (using the FAS-2PL spectrometer) both deviate from the PSIM curve as the Fe particle size exceeds -1 pm. The rotatingdisk electrode atomic emission spectrometers (AE35U-3 and A/E35U-1) are intermediate in capability and the dc argon plasma spectrometer (SMI111) is slightly more efficient. The particle size detection limitation poses the serious potential problem that an aircraft engine undergoing severe weiu may not be identified.
Sample Introduction Systems
1
Figure 2. Comparison of the detection of Fe particles by various spectrometers with the acid dissolution method (PSIM) 1088A
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
One of the major reasons for particle size detection limitations of these analytical spectroscopic techniques is the inefficiency of the sample introduction systems used in many flame AA and plasma atomic emission spectrometers (7).Many of these sample introduction systems incorporate nehulize-spray chamber systems, which were designed for transporting bomogeneous samples and are quite inefficient when applied to samples that contain suspended metal particulates. The larger wear particles collect on the walls of the spray chamber and are washed into the sample drain or waste container. Only the relatively small particles find their way into the Sample excitation region. The rotatingdisk electrode is also an inefficient particle transport system because its efficiency depends on the settling rate of particles (12).The settling rate of particles depends on the lubricant’s viscosity as well as the particle’s density. Therefore, heavy metal particles, such as Ag and Mo,settle rapidly and are not transported to the source, whereas 10-20-rm particles of the
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Figure 3. Comparison of percent recovery of metal particles with and without acid
lighter elements, such as Mg and AI, are transported to the source where they can be detected. In addition, the power of the spark-arc source is not sufficient to vaporize metal particles in the oil matrix (12).
AAS Flame AAS as currently employed by the Air Force for wear metal determinations is accomplished in a sequential manner and is not well suited to heavy sample workloads. Furthermore, the present instrumentation is not rugged enough to be transported easily and requires shipment and handling of flammable gases. These factors, along with the severe particle size dependency of flame AAS, render the technique unreliable for accurate analysis of wear metal particles of the sizes expected from severe wear. One technique, the electrothermal analyzer as applied in graphite furnace AAS, permits the physical introduction of the sample directly into the 10901
furnace by means of a micropipet. This technique has been found to he capable of determining metal particles of sizes in the 2630-pm range (13). Because of the deficiencies of flame AAS, the Air Force is currently s u p porting the development of a rugged simultaneous multielement AA spectrophotometer based on electrothermal atomization for use as a portable wear metal analyzer (14,15). This device is used primarily for oil sample analysis of aircraft deployed at locations away from fixed laboratory spectrometers.
Particle-Sire-Independent Techniques Inasmuch as severe wear can produce metal particles of sizes that cannot be detected hy these spectrometers, alternate approaches are required. One approach to this problem has been the development of particlesize-independent techniques for wear metal analysis by flame AAS and AJ3S
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1984
( 4 4 8). These methods simply require shaking a small amount of an acid solution with the used oil sample prior to spectrochemical analysis. Initial requirements for particlesize-independent analysis surfaced approximately 10years ago when several fighter aircraft experienced gas turbine engine failures that were undetected by oil analysis. The reason for these engine failures was abnormal wear of Ti components within the oilwetted system of the engines. It was soon discovered that direct flame AA analysis of the oil samples from these engines was not capable of detecting this severe Ti wear situation. A simple procedure was developed whereby a diluted oil sample containing Ti wear particles is shaken for 10 s with a small quantity of a mixture containing HCI and HF (4). The acid contact results in dissolution of the Ti particles. Only 0.15 mL of acid is required for a 15-g diluted oil sample, and no phase separation is apparent. The oil sample containing the dissolved Ti and excess acid can then be analyzed directly with reliable results by flame AAS for Ti using a nitrous oxide-acetylene flame. A similar procedure was later developed for Mo determinations (5). It soon became evident that wear particle detection was a general problem that was not specific to Ti or Mo and that a more universal procedure was required. The most widely used of these approaches ( 8 )has application to aircraft, automobile, and diesel truck engine oils as well as aircraft hydraulic fluids for the particle-size-independent analyses of 12 common wear metals. Figure 3 demonstrates the effectiveness of this multielement acid dissolution method (MADM) by comparing the percent recoveries of suspensions that contain 13 metal powders by direct analysis of the suspensions (no acid) and by MADM analysis. The suspensions were prepared using 325mesh (44-pm) or 200-mesh (74-rm) metal powders. All data were obtained using the direct current plasma (DCP) spectrometer. The results represent triplicate analyses of the suspensions. The shaded area represents the standard deviation of the analyses. Table I1 compares data for two aircraft engine oil samples obtained directly (without acid dissolution of the wear particles) using flame AA, DCP, and RDE atomic emission with the results obtained on the same samples using MADM and DCP. In the table, the RDE values are artificially higher because they include the influence of the previously described interelement and matrix effects. A specific embodiment of the acid dissolution approach is found in a por-
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ANALYTICAL CHEMISTRY,
VOL. 56, NO. 9. AUGUST 1984
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1091 A
Table II. Comparison of Direct Analysis vs. Multielement Acid Dissolution Method (MADM) for Authentic Aircraft Engine Oil Samples w.st .mM.
AQ AI
cu Fe
Ma
M
Dcp
RDE
YIDM DCP
1.3 1.6 5.6 21.2 9.8
1.8 2.1 7.0 28.9 11.5
1.0 4.3 15.6 61.3 11.0
4.9 8.0 21.0 81.3 25.3
Engine: 156 Aircraft C-130E Hours since ovhaul: 1123 Hours since oil change: 400
table, easy-to-operate, low-cost colorimetric Fe kit (16-18). This kit, which provides a readout directly in ppm of Fe, was developed and tested to provide a goho-go flight decision based on iron wear metal content of the lubricating oil of aircraft deployed at remote locations.
Standard Selection Another factor of primary concern in spectrometric oil analysis is the selection of a standard. Traditionally, metalloorganic compounds have been used as standards for spectrometric wear metal analysis. Care should be taken to match the standard oil matrix with that of the sample to reduce matrix effects on the analysis. Problems can occur particularly in plasma spectrochemical analysis when metalloorganic standards are used. Although the viewing position of the plama is normally fixed, it has been
D*.sl Wh.*
AA
AQ
4.2 14.4 136.8 Fe 105 Mo 0.1 48.9 Engine: J-85 Aircraft T-38
cr cu si
RDE
5.7 27.3 271.0 286 1.6
6.5 21.5 314 361 7.5 .. 69.9
50.9
~
YIDM DCP
10.0 38.1 546 394
....
17 7
71.6
Hours since owhaul: 2208 Hours since oil chanw: 1
found that the maximum emission POsition of the DCP can be auite different for a given metal when comparing the emission from a metalloorganic compound with, for example, a powdered form of the same metal, as seen in Figure 4 for the case of Fe and AI (2).Even when using high-purity metal powders as standards, the maximum emission region of the DCP is at a higher position for larger particles relative to smaller particles of the same metal (2).These factors also contribute to difficulty in achieving accuracy when determining wear metals of unknown size distribution in a used oil sample. This can he a no-win situation for the analyst. Does the operator optimize the plasma height for the metalloorganic standard or for each sample, which may contain different particle size distributions of different metals? Even when using high-purity, small-particle-size metal
FigZrO 4. Effect of dc plasma ObSeNatiOn height on metal particle detection 1082A
DCP
ANALYTICAL CHEMISTRY, VOL. 56. NO. 9, AUOUST 1984
powders suspended in oil as standards, the viewine Dosition for maximnm emission ofasample may he different depending on the particle size distribution of that sample (2). Unless the acid dissolution method is used, great care must be taken in the selection of any standard. All of these factors substantially reduce the appeal of direct sample analysis.
Sampling It is imperative that proper sampling techniques he used when an oil sample is taken, to ensure that the sample removed is representative of the total quantity of lubricant and wear in the system. The best analysis is worthless unless the sample has heen removed from the engine in the proper manner. Once the sampling is complete, it is essential that the integrity of the original sample he maintained until all analyses are complete.
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