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Energy and Earth Science 
Vol. 2, No. 2, 2019 

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ISSN 2578-1359 (Print)   ISSN 2578-1367 (Online) 

54 
 

The Investigation on Rheology and Microstructure for Lithium 

Complex Grease 

Mao Jingjing1*, Li Jianming1, Qiu Jianwei1, Liu Lijun1, Wang Yue1 & Zhang Lijiuan1 
1 Petrochina Lanzhou Lubricating Oil R&D Institute, Lanzhou, China 
* Mao Jingjing, E-mail: maojingjing_rhy@petrochina.com.cn 

 

Received: July 25, 2019   Accepted: August 20, 2019    Online Published: September 4, 2019 

doi:10.22158/ees.v2n2p54         URL: http://dx.doi.org/10.22158/ees.v2n2p54 

 

Abstract 

The rheological characteristic parameters of five lithium complex lubricating greases were determined. 

The comparison of rheological characteristic parameters and microstructure of the five greases was 

made. The results showed that the knowledge of the rheological properties of lubricating greases may 

contribute to reflect the change of the thickener structure. The flow transition index characterized the 

breaking behavior of inner structure of grease, the greater this index, the better that soap fiber structure 

of grease; The damping factor was shifted from a medium range towards a lower value which resulted in 

brittle character of sample. At a constant shear rate, the soap fiber structure of complex lithium grease is 

dense and uniform, apparent viscosity decline rate is small. In this sense, it is relevant to understand how 

the development of the soap fiber structures in the grease contributes to several functional and 

rheological properties of lubricating greases. 

Keywords  

lithium complex grease, rheological property, viscoelasticity, microstructure 

 

1. Introduction 

The rheology of grease refers to the flow and deformation characteristics of grease when subjected to 

shear stress. Grease is a colloidal structure with significant non-Newtonian fluid properties. It can 

maintain a certain shape like a solid when it is not subjected to shear stress, that is, it will not flow 

when it is at rest. When subjected to weak shear stress, it will be elastically deformed; After 

removing shear stress, it can return to its original position and shape, exhibiting the elastic properties 

of the solid. When the applied shear stress is large enough, the grease deforms and flows, and thus 

can no longer automatically return to the original position and shape, so the starting torque of the 

grease in the machinery is larger than that of the liquid lubricating oil. During the flow of the grease, 

as the shear stress increases, the soap fibers are aligned, which reduces the apparent viscosity of 

grease. In the case of extremely high shear stress, the grease flows like a Newtonian fluid, and the 



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viscosity remains constant and does not change with the shear rate. The rheological properties of the 

grease are directly related to the microstructure of the system and the interaction between the internal 

materials, and are the macroscopic representation of the internal microstructure of the system. 

Therefore, studying the rheology of grease is of great significance for the production and application 

of grease. 

In recent years, more and more researchers have used different methods to study the rheological 

properties of grease. Meng et al. studied the rheological properties of lithium grease based on 

thixotropy at different shear rates, different time and different temperatures. The rheological model 

with structural parameters was fitted from the experimental results. The model can describe the yield 

stress, shear dilution and thixotropy of the tested grease. Guo et al. studied the rheological properties 

of semi-fluid lithium grease and complex lithium grease at different consistency and temperature, 

and discussed the effects of the type, content, temperature and shear rate of thickeners on their 

rheological properties. Wang et al. studied the effects of structure and content of thickener, 

temperature, base oil viscosity and shear time on rheological parameters were investigated by HAAK 

rheometer. The results show that the strength limit of grease decreases with the increase of 

temperature, but the change of plastic viscosity and plastic index is irregular. Delgado et al. studied 

the relationship between rheological properties and microstructures in the preparation of lithium 

grease. J.M. Franco et al. studied the mixed rheological properties in the production of grease. J.E. 

Martin-Alfonso et al. studied the effect of a polymer on the rheological properties of greases. Yeong 

et al. studied the steady flow and viscoelasticity of greases with different thickener content. However, 

the test instruments and methods used by researchers are different, and the rheological performance 

test results are also described in different ways. The research contents are mainly focused on 

thixotropy, yield stress and apparent viscosity of lubricating grease rheology. The viscoelastic 

characteristic parameters of lubricating grease and their relationship with microstructure have not 

been reported.  

In 2007 and 2011, the German standard revised and published the DIN 51810-1 “Determination of 

the shear viscosity of greases using a rotational viscometer—Part 1: Cone and plate inspection 

systems”, DIN 51810-2 “Determination of lubrication Rheological properties—Part 2: Determination 

of the flow point using an oscillating rheometer with a parallel plate detection system. The standard 

specifies the test methods and conditions for grease rheology, but the standard is not used to describe 

how the test results should be understood and applied. In addition, therefore, this paper applies and 

discusses the standard method through literature and data accumulation. The rheological tests of 

several lithium complex grease samples were carried out, and the test results were analyzed. The 

microstructure of the samples was compared with the samples. It is believed that by testing the 

rheological parameters of lubricating grease, it has certain guiding significance for the development 

and application of grease products. 

 



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2. Experiment Part 

2.1 Test Equipment 

The electron micrograph was taken by a field emission scanning electron microscope (FE-SEM) 

manufactured by JEOL, model number JSM-6701F. The rheology test uses Anton Paar’s MCR301 

rheometer with air bearings, motors and optical encoders. It can be extended to different temperature 

control units with control stress and control rate functions. The oscillating mode experiment used a 

parallel plate test system with a diameter of 25 mm. The rotation mode experiment used a cone and 

plate test system with a diameter of 25 mm and an angle of 1.0 °C. Test temperature accuracy ± 0.1 

°C. 

2.2 Test Methods 

Refer to DIN 51810-1 and DIN 51810-2. 

2.3 Preparation and Performance of Grease Samples 

The base oil and 12-hydroxystearic acid were added to the kettle and stirred, and the mixture was 

heated to 80 °C to dissolve. After 90-100 °C, a lithium hydroxide aqueous solution was added and 

saponified for 2 hours. Dehydrated at 130 °C for 1 h. The aqueous solution of lithium hydroxide and 

azelaic acid was added to carry out a reaction for 1 hour, and the mixture was heated to 160 °C for 1 

hour, and then the remaining base oil was added, and the temperature was raised to 210 °C for 10 

minutes. Final cooling, addition of additives and homogenization. The properties of the five samples 

are shown in Table 1. Samples 1 and 2 were commercial greases products, and the others were 

laboratory prepared samples. 

 

Table 1. Basic Properties of Five Lithium Complex Grease 

Test items sample1 sample
2

sample
3

sample
4

sample 
5 Test Methods

Worked penetration/0.1mm 279 232 213 256 225 ISO 2137 

Prolonged worked penetration 
/0.1mm 295 250 233 272 267 ISO 2137 

Dropping Point/℃ 304 275 317 316 315 ISO 6299 

oil separation /% 2.52 0.5 0 3.02 2.31 ASTM 
D6184 

 

3. Results and Discussion 

3.1 Comparison of Microstructure 

The microstructures of the five lithium complex grease samples were compared, and the electron 

micrographs of the photographs are shown in Figure 1. It can be seen from the figure that the soap 

fiber network structure of the five samples has obvious differences. The soap fiber of sample 1 is 

slender, the diameter is the most uniform, and almost all the fibers exhibit a clear double helix-like 

structure, the whole fiber network is dense and interlaced. The fiber morphology and regularity of 

samples 2 and 3 were slightly worse than those of sample 1, and there were relatively coarse fibers. 



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The soap fiber of sample 4 was uneven in diameter and length, and part of the soap fiber had no 

double helix structure. Sample 5 had the worst morphology of soap fibers, loose network structure, 

different diameter and length of soap fibers, and lumpy and flaky soap fibers. The difference in 

microstructure of the lithium complex grease sample will result in a change in macroscopic 

rheological properties. 
 

sample 1                        sample 2 

sample 3                        sample 4 

 
sample 5 

Figure 1. SEM Photograph of Five Lithium Complex Greases 

 

3.2 Comparison of Viscoelasticity  

Grease is a semi-solid material that is both viscous and elastic. The oscillating experiments of five 

lithium complex grease samples were carried out by rheometer, and the relationship between the storage 

modulus G’ and the loss modulus G’’ as the shear stress increased was obtained, as shown in Figure 2 and 

Figure 3, The storage modulus G’ represents the elastic part of the viscoelasticity of the grease, indicating 

that the stress energy is temporarily stored in the test and can be recovered. The loss modulus G' 

represents the viscous portion of the viscoelasticity of the grease, indicating that the energy used for the 

initial flow is an irreversible loss and the energy is converted to shear heat. At 25°C and -40°C, the 

modulus of the five samples decreased with the increase of stress. When the stress is small, G’ is greater 

than G’’, indicating that the samples have more similar structural features to solids. The point at which 

the storage modulus G’ begins to decrease is usually defined as the end point of the linear viscoelastic 

region (LVE), which determines the maximum deformation that the sample can withstand before the 



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internal structure is destroyed. The shear stress τy at this time is also called yield point. In the linear 

viscoelastic region, the grease has gel characteristics, physical or chemical cross-linking network 

structure, G’ and G’’ can characterize the structural strength and viscosity of the grease, and Thickener is 

the main factor affecting the structure of grease. It can be seen from Figure 2 that the modulus values of 

the five samples with higher consistency at normal temperature are also higher, and the corresponding 

yield stress τy is also larger, which is consistent with the actual use of the grease. When the mechanical 

parts rotate, generally grease with a thicker consistency require a larger external force to deform, so the 

rotational torque is also higher. As the shear stress increases, the modulus decreases. When the shear 

stress τ>τy, the sample enters the yielding zone. The soap fiber structure of the lithium complex grease 

begins to be destroyed, and the soap fibers begin to be rearranged, but at this time the sample still has gel 

characteristics, only partial deformation can be recovered; At higher shear stress, The lines G’ and G’’ 

intersect at one point, also known as the flow point, the stress at G’=G’’ is taken as the flow stress τf, 

indicating that the sample begins to flow at this time, and the fiber network structure of the lithium 

complex grease is greatly destructed, when the shear stress τ>τf, the viscosity characteristic of the sample 

is greater than the elastic characteristic, G’’>G’, indicating that the sample is in a liquid state and exhibits 

flow behavior. 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2. The Storage and Loss Modulus for Greases（25℃） 

 

 

 

 

 

 

 

 

10
2

10
3

10
4

10
5

10
6

Pa
G'

G''

1 10 100 1,000 10,000Pa
Shear Stress 

Sample 1  Sample 1[LVE-R]  Sample 1[Flow Point] 

Sample 2  Sample 2[LVE-R]  Sample 2[Flow Point] 

Sample 3  Sample 3[LVE-R]  Sample 3[Flow Point] 

Sample 4  Sample 4[LVE-R]  Sample 4[Flow Point] 

Sample 5  Sample 5[LVE-R]  Sample 5[Flow Point] 



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Figure 3. The Storage and Loss Modulus for Greases（-40℃） 

 

The calculated viscoelastic characteristic parameters are shown in Table 2 and Table 3. The stress value 

τf of the flow point is one of the important characteristics of the grease, and has significance in the 

application of the grease. The flow stress of the five samples at -40°C was significantly increased 

compared with 25°C, indicating that the fluidity of the lithium complex grease deteriorated at low 

temperatures. Comparing sample 2 and sample 3 with close consistency, it was found that the flow stress 

of sample 3 was much higher than that of sample 2 at -40°C, indicating that the low temperature fluidity 

of sample 2 was better. The flow transition index τf/τy is also an important rheological characteristic 

parameter, which can describe the failure behavior of the internal structure of grease. The closer the value 

is to 1, the more the grease exhibits brittleness, and the soap fibers are more susceptible to breakage when 

sheared. It can be seen from Table 2 that the τf/τy of the five samples differs greatly at 25°C, and the τf/τy 

value of the sample 1 is the largest, indicating that the soap fiber structure is the most stable and tough, 

compared with the electron micrograph in Figure 1, The soap fiber of sample 1 has the most uniform 

morphology and the most compact network structure. The τf/τy values of sample 2 and sample 3 are 

relatively close, and the morphology and uniformity of the soap fibers are similar in terms of 

microstructure. The τf/τy value of sample 4 was much lower than that of sample 1, and the corresponding 

microstructure was also significantly deteriorated, indicating that the soap fiber structure was more easily 

destroyed and the stability was poor when sheared by stress. The τf/τy value of sample 5 was the smallest, 

and the grease structure was most easily destroyed. Compared with the microstructure, the soap fiber 

morphology of sample 5 was also the worst. Therefore, comparing the flow transition index τf/τy of five 

samples at room temperature and the structure of soap fiber, it is found that there is a certain relationship, 

and the poorer the morphology of the soap fiber structure, the smaller the value of τf/τy. It can be seen 

from Table 3 that the grease will become hard and brittle at -40°C due to the extremely low temperature, 

and the τf/τy values of the five samples are significantly reduced and the difference is not large. It is 

indicated that the flow transition index τf/τy of the lithium complex grease can be tested at room 

10
3

10
4

10
5

10
6

10
7

Pa
G'

G''

10 100 1,000 10,000 100,000Pa
Shear Stress 

Sample 1  Sample 1[LVE-R]  Sample 1[Flow Point] 

Sample 2  Sample 2[LVE-R]  Sample 2[Flow Point] 

Sample 3  Sample 3[LVE-R]  Sample 3[Flow Point] 

Sample 4  Sample 4[LVE-R]  Sample 4[Flow Point] 

Sample 5  Sample 5[LVE-R]  Sample 5[Flow Point] 



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temperature, which can better reflect the stability of the internal structure of the sample. In addition, the 

loss factor or damping coefficient tan δ=G’’/G’ can also be used as an added value for grease 

performance description. The loss coefficient values in the linear viscoelastic region are shown in Table 2 

and Table 3. The small loss factor value indicates that the sample is brittle and the loss factor is large, 

indicating that the sample is easier to spread. It can be seen from Table 2 that the sample has the largest 

tanδ value at room temperature, and the soap fiber is finer and uniform in terms of microstructure, so that 

it is easier to spread and enter the raceway surface. The tanδ values of sample 2, sample 3, and sample 4 

were substantially at one level. Sample 5 has the smallest tanδ value, and its soap fiber structure is also 

the worst. The uneven soap fiber makes it difficult to enter the raceway, and the sample is brittle. At 

-40°C, the tan δ values of the five samples increased to different extents, indicating that the brittleness of 

the grease became larger at low temperatures, but the relationship between the tan δ value and the 

microstructure of the sample was not obvious, indicating the rheology of grease is not only related to 

thickeners, but also factors such as base oil at low temperature. 

 

Table 2. Viscoelastic Characteristic Parameters for Lithium Complex Greases（25℃） 

Viscoelastic characteristic parameter 
Sample number 

1 2 3 4 5 

Yield point τy /Pa 33.2 79.1 178 83.5 335 

Flow point τf /Pa 809.5 1238 2192 616.1 1873 

Flow transition index τf/τy 24.38 15.65 12.31 7.38 5.59 

Loss factor tanδ 0.311 0.233 0.236 0.258 0.135 

 

Table 3. Viscoelastic Characteristic Parameters for Lithium Complex Greases（-40℃） 

Viscoelastic characteristic parameter 
Sample number 

1 2 3 4 5 

Yield point τy /Pa 10200 562 3720 3070 571 

Flow point τf /Pa 35570 1814 23390 10470 1926 

Flow transition index τf/τy 3.49 3.23 6.29 3.41 3.37 

Loss factor tanδ 0.345 0.3745 0.2595 0.407 0.473 

 

3.3 Comparison of Shear Viscosity 

Viscosity is a property of a material that undergoes flow and irreversible deformation under stress. The 

relationship between the shear stress τ of Newtonian fluid and the shear rate   is =    . While the 

grease is a non-Newtonian fluid, the shear stress and shear rate are not proportional, so the shear viscosity 

η can only be determined under the specified test conditions. The viscosity of the five lithium complex 

grease samples was measured at a constant shear rate as shown in Figure 4.  



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Figure 4. The Shear Viscosity Changes for Lithium Complex Greases 

 

Since the grease is a shear-thinning fluid, the viscosity of the five samples decreased slightly under 

constant shear at 5 minutes. The initial viscosity A  and the final viscosity E  in the test period and the 

viscosity decrease rate rel are recorded from the graph, and are calculated according to the 

equation
-

100A E
rel

A

 



  , expressed in units of percentage. As shown in Table 4, it can be seen that the 

viscosity drop rate of the sample 1 is the smallest, and a certain viscosity can be maintained after shearing, 

and the soap fiber structure is the most stable. The viscosity reduction rates of sample 2, sample 3, and 

sample 4 were substantially at one level, and the stability of the soap fiber structure was not much 

different. The viscosity of the sample 5 decreased the fastest, and the viscosity decreased by nearly half 

after 5 minutes of shearing, indicating that the shear resistance was poor. Therefore, the denser and more 

uniform the soap fiber structure of the lithium complex grease, the better the viscosity is maintained 

during the shearing process, and the poorer the soap fiber structure, the faster the viscosity decreases 

during shearing. If the viscosity drops too fast during use, it will be lost or smashed, which will 

eventually lead to lubrication failure. 

 

Table 4. The Shear Viscosity for Lithium Complex Greases 

Shear viscosity 
Sample number 

1 2 3 4 5 

Initial viscosity ηA /Pa·s 4.54 6.14 6.90 3.51 5.57 

Final viscosity ηE /Pa·s 4.35 5.38 5.91 3.14 3.09 

Relative viscosity drop rate ηrel /% 4.17 12.3 14.3 10.5 44.5 

 

4. Conclusion 

(1) By determining the rheological characteristic parameters of the grease, the stability and the degree of 

damage of the lithium complex grease structure can be described, which has certain guiding significance 

for the development and use of the product. The rheological properties of lithium complex greases are 

related to the microstructure of the grease and are the macroscopic representation of the internal 

1

10

Pa·s


5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10min
t

Sample 1  Sample 2  Sample 3  Sample 4  Sample 5  



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microstructure of the grease. The lithium complex grease with excellent performance should have a 

dense and uniform, staggered three-dimensional network structure, and the soap fiber exhibits a distinct 

double helix structure. 

(2) The viscoelastic characteristic parameters are related to the microstructure of the grease. The flow 

transition index τf/τy is an important rheological characteristic parameter that can describe the failure 

behavior of the internal structure of the grease. The poorer the soap fiber structure of the lithium complex 

grease, the smaller the value of τf/τy, indicating that the soap fiber is more likely to break under the action 

of shearing force. The loss factor tan δ can be used as an added value for grease performance description. 

The hourly loss factor indicates that the sample is brittle and the loss factor is large, indicating that the 

sample is easier to spread. 

(3) At a constant shear rate, the denser and more uniform the soap fiber structure of the lithium complex 

grease, the better the viscosity is maintained during shearing, the smaller the relative viscosity drop rate, 

and the worse the soap fiber structure. The faster the viscosity decreases during the shearing process, the 

greater the rate of decrease in relative viscosity. 

 

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