MolaKule
Staff member
A New Theory of ZDDP Tribofilm Formation
by Molekule
A new theory of tribofilm formation by the ZDDP additive has been put forth in a recent industry paper and it appears to have substantial experimental support[1].
Here, I will attempt to distill the paper for the general BITOG reader and define certain words and phrases as we progress.
Subject Content: Short History of ZDDP, Prior Theories of ZDDP film Formation, The New Theory and Verification of ZDDP Tribofilm Formation, Summation.
Keywords: Tribofilms, thermal films, mechanochemistry, stress-promoted thermal activation, anti-wear, tribochemistry, flash temperature,
Short History of ZDDP
First used as a lubricant anti-oxidant, it was later found that ZDDP had a welcome additional benefit as an Anti-Wear (AW) agent. ZDDP has been in use in various forms for the past 60 years in all types of lubricants. During that time, many theories have been presented as to how this film forms and how it reacts with surfaces in motion and under loads.
ZDDP, dialkyl-dithiophosphate or diaryl-dithiophosphate, is a molecule consisting of phosphorus, sulfur, and zinc atoms. Structurally, this molecule has one phosphorus atom on each end with four sulfur atoms linked to a single zinc atom in the middle.
On rubbing surfaces, a thick film of amorphous zinc phosphate is found. ZDDP films are rough and form circular or elongated pads which are 200 nm thick. The ZDDP film is believed to control wear by limiting direct contact of two surfaces in motion, preventing adhesion and transient contact stresses. (amorphous – a non-crystalline solid).
When surfaces rub together in a lubricant containing ZDDP, films are generated quickly at temperatures less than or equal to 25C. These are what we call, “Tribofilms.” At bulk oil temperatures above 150C, ZDDP will form films on surfaces even in the absence of motion or near contact. These films are what we call, “Thermal Films.”
Prior Theories of ZDDP film Formation
We will in this section discuss the four main and prior theories of ZDDP film formation: flash temperature rise, pressure, triboemission, and surface catalysis.
Flash temperature theory:
When solid surfaces come in contact, heat is generated which results in local and transient temperature rise. These are called “flash temperatures.” Flash temperatures are most prevalent in transmission wet and dry clutches, for example. However, ZDDP films have been shown to form at very low sliding speeds and low temperatures as noted above.
High Pressure film forming:
In high pressure contact areas such as gear teeth and other non-conforming contacts, pressures can be very high and it has been proposed that high pressures can produce “cross-linking” in the phosphate linkage network. However, once again, high pressure studies have shown no changes in ZDDP films up to 21 GPa.
Triboemission:
This topic involves the emission of energetic particles such as photons, electrons, ions, and even X-rays during localized stresses and deformations. It is believed that the emission of these energetic particles gives rise to the reaction of ZDDP films on surfaces. But this theory cannot be confirmed in lubricated contacts.
Surface Catalysis:
This theory promoted the catalytic reaction of various iron (ferrous) species with the phosphorus and sulfur atoms, resulting in ferrous sulfates and ferrous phosphates in the formation of ZDDP films. Many studies have been executed which show various chemical species in the film, but it cannot be determined if this is the “cause” of film formation, or simply the result of asperity mixing.
The New Theory and Verification of ZDDP Tribofilm Formation:
This theory is called, “Stress-Promoted Thermal Activation,” and is the main topic of this paper. This theory involves the modern developed field of, “mechanochemistry.” Mechonochemistry is where mechanical forces, such as shear-stresses, raises the activation energy of a chemical mix or a set of chemical species to promote a reactant-to-product result. Mechanochemistry is also defined as the, “mechanical activation of covalent bonds”.
(Activation energy - the minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction; Covalent chemical bonds - the sharing of a pair of valence electrons by two atoms, Such bonds lead to stable molecules; Valence electrons are the electrons in the outer shell of an atom. The valence electrons are the ones involved in forming bonds to adjacent atoms).
Since shear stresses are always present in mechanical systems, it makes sense to examine more closely the thermal activation of chemical systems by shear stresses.
If applied shear stresses are high enough, then ZDDP film formation could occur even in the full film lubrication regime, and in the absence of high pressures.
Through the experimental method of using a track-on-ball machine, the authors verified that shear stress of ZDDP solutions forms a tribofilm and the rate of film formation depends on the magnitude of the shear stress in agreement with the shear-stress thermal activation model.
For Extreme Pressure additives such as predominately P/S additives, or for zinc-free AW additives such as polymer esters, it is doubtful that shear-stress thermal activation is the cause of film forming.
Summary:
The results of this experiment could lead to a better understanding of film formation by not only ZDDP, but by other additives in the Performance Package.
If indeed shear stress related activation is the primary causation of various films, then improved AW and friction reducing additives could be “fine-tuned” in the molecular sense, and improved for the Low Viscosity engine and transmission oils.
1. Spikes and Zhang, On the Mechanism of ZDDP Antiwear Film Formation, Tribology Letters, 63:24, 2016.
by Molekule
A new theory of tribofilm formation by the ZDDP additive has been put forth in a recent industry paper and it appears to have substantial experimental support[1].
Here, I will attempt to distill the paper for the general BITOG reader and define certain words and phrases as we progress.
Subject Content: Short History of ZDDP, Prior Theories of ZDDP film Formation, The New Theory and Verification of ZDDP Tribofilm Formation, Summation.
Keywords: Tribofilms, thermal films, mechanochemistry, stress-promoted thermal activation, anti-wear, tribochemistry, flash temperature,
Short History of ZDDP
First used as a lubricant anti-oxidant, it was later found that ZDDP had a welcome additional benefit as an Anti-Wear (AW) agent. ZDDP has been in use in various forms for the past 60 years in all types of lubricants. During that time, many theories have been presented as to how this film forms and how it reacts with surfaces in motion and under loads.
ZDDP, dialkyl-dithiophosphate or diaryl-dithiophosphate, is a molecule consisting of phosphorus, sulfur, and zinc atoms. Structurally, this molecule has one phosphorus atom on each end with four sulfur atoms linked to a single zinc atom in the middle.
On rubbing surfaces, a thick film of amorphous zinc phosphate is found. ZDDP films are rough and form circular or elongated pads which are 200 nm thick. The ZDDP film is believed to control wear by limiting direct contact of two surfaces in motion, preventing adhesion and transient contact stresses. (amorphous – a non-crystalline solid).
When surfaces rub together in a lubricant containing ZDDP, films are generated quickly at temperatures less than or equal to 25C. These are what we call, “Tribofilms.” At bulk oil temperatures above 150C, ZDDP will form films on surfaces even in the absence of motion or near contact. These films are what we call, “Thermal Films.”
Prior Theories of ZDDP film Formation
We will in this section discuss the four main and prior theories of ZDDP film formation: flash temperature rise, pressure, triboemission, and surface catalysis.
Flash temperature theory:
When solid surfaces come in contact, heat is generated which results in local and transient temperature rise. These are called “flash temperatures.” Flash temperatures are most prevalent in transmission wet and dry clutches, for example. However, ZDDP films have been shown to form at very low sliding speeds and low temperatures as noted above.
High Pressure film forming:
In high pressure contact areas such as gear teeth and other non-conforming contacts, pressures can be very high and it has been proposed that high pressures can produce “cross-linking” in the phosphate linkage network. However, once again, high pressure studies have shown no changes in ZDDP films up to 21 GPa.
Triboemission:
This topic involves the emission of energetic particles such as photons, electrons, ions, and even X-rays during localized stresses and deformations. It is believed that the emission of these energetic particles gives rise to the reaction of ZDDP films on surfaces. But this theory cannot be confirmed in lubricated contacts.
Surface Catalysis:
This theory promoted the catalytic reaction of various iron (ferrous) species with the phosphorus and sulfur atoms, resulting in ferrous sulfates and ferrous phosphates in the formation of ZDDP films. Many studies have been executed which show various chemical species in the film, but it cannot be determined if this is the “cause” of film formation, or simply the result of asperity mixing.
The New Theory and Verification of ZDDP Tribofilm Formation:
This theory is called, “Stress-Promoted Thermal Activation,” and is the main topic of this paper. This theory involves the modern developed field of, “mechanochemistry.” Mechonochemistry is where mechanical forces, such as shear-stresses, raises the activation energy of a chemical mix or a set of chemical species to promote a reactant-to-product result. Mechanochemistry is also defined as the, “mechanical activation of covalent bonds”.
(Activation energy - the minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction; Covalent chemical bonds - the sharing of a pair of valence electrons by two atoms, Such bonds lead to stable molecules; Valence electrons are the electrons in the outer shell of an atom. The valence electrons are the ones involved in forming bonds to adjacent atoms).
Since shear stresses are always present in mechanical systems, it makes sense to examine more closely the thermal activation of chemical systems by shear stresses.
If applied shear stresses are high enough, then ZDDP film formation could occur even in the full film lubrication regime, and in the absence of high pressures.
Through the experimental method of using a track-on-ball machine, the authors verified that shear stress of ZDDP solutions forms a tribofilm and the rate of film formation depends on the magnitude of the shear stress in agreement with the shear-stress thermal activation model.
For Extreme Pressure additives such as predominately P/S additives, or for zinc-free AW additives such as polymer esters, it is doubtful that shear-stress thermal activation is the cause of film forming.
Summary:
The results of this experiment could lead to a better understanding of film formation by not only ZDDP, but by other additives in the Performance Package.
If indeed shear stress related activation is the primary causation of various films, then improved AW and friction reducing additives could be “fine-tuned” in the molecular sense, and improved for the Low Viscosity engine and transmission oils.
1. Spikes and Zhang, On the Mechanism of ZDDP Antiwear Film Formation, Tribology Letters, 63:24, 2016.
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